Native speakers who are shy and quiet aren't as eloquent as talkative guys either, and just because you've been exposed to the standard dialect of your native language in media since you were born doesn't mean you can fake the neutral accent like professional voice talents. The same goes for writing. The more you write, the better you're at it, regardless of whether it's your first or second language. Without practice, your writing can't be good.
So, even with native proficiency in a language, you need practice to an extent. If your grammar is shaky and your vocabulary is tiny, it must be much harder. So if you completely ignore speaking/writing practice of any kind, your output skills won't improve much.
But you can't improve your output skills without developing good listening and/or reading skills first. And it's absolutely normal that your passive skills outperform your active skills by a huge degree. Your average guy may be able to enjoy great novels and appreciate believable acting in your native language, but they're not as good at writing or acting as professional novelists or actors. Not even close. You can tell the quality of writing, speech, acting, etc., though not as reliably as experts in each field. But you can't write or deliver speech effectively without training. And if you can't see the difference between good and poor performances in the first place, there's no way you can be good at production.
I do think 4 moths is too short to acquire sufficient listening and reading skills. I don't think you can have foundations decent enough to develop great output skills on top of them in such a short period of time.
But if you do believe you've developed good enough listening/reading skills and are still struggling to improve output skills, maybe that's because you're only good at listening/reading "comprehension." I don't think good comprehension alone helps much when it comes to developing linguistic intuition that allows you to tell if a given sentence is right or wrong, which I think is a necessary skill to be proficient in speaking and writing.
You might be wondering what the difference is between good listening/reading and good comprehension. I think the difference is how well you can detect errors. For example, you listen to a pair of complete sentences in Japanese where one is grammatically perfect and sounds natural and the other has a simple error such as particle は swapped with が. With good listening skills, you should be able to catch the nonstandard use of the particle just like native speakers would. But you can perfectly comprehend the intended meaning if you fail to notice the error. So, you can be pretty good at comprehension without good listening skills.
Noticing unusual usages of particles and other little grammar rules isn't the only thing that separates good listening from good comprehension. You may be good at comprehension but suck at listening if you are not good at catching slight differences in prosody, intonation, tone of voice, and such. Just because you have good comprehension doesn't mean you can hear accents, dialectical differences, and the like either.
If you don't notice the difference between 今日はどこ行こっか vs. 今日どこ行こうか when spoken to you or when reading them quickly, you won't acquire linguistic intuition native speakers have because to you they're exactly the same when they're not to native speakers. The intended meanings are pretty much the same in this example, but a slight change can alter the meaning of a sentence drastically too.
So, in short, your comprehension can be pretty good even if you always ignore certain details. And if you always ignore them, you won't have good listening/reading skills that form the foundations of your output skills.
Obviously, your speaking and writing can only be as good as your listening and reading at best. And if you're measuring your passive skills by your comprehension level, you might feel like your active skills are always shaky and won't improve by exposure because you're practically ignoring details when listening and reading but you probably care about details more when speaking and writing. Of course, you can speak or write fluently if you're ok with poor grammar, pronunciation, spelling etc. With a little practice, you'll reach the level your true listening/reading skills allow. It's just you can't expect great results just because your comprehension level is great.
Improving listening (not comprehension) and paying more attention to details when reading is an obvious solution to this problem. If you're having trouble hearing certain sounds or sequences of sounds, there are lots of promising methods out there. Some are just anecdotal, and others are verified by serious research. You can pick your favorite method and see if it works for you.
Learning grammar can be helpful to an extent as well. It only improves comprehension as far as listening goes, but you can definitely benefit from solid grammar knowledge when writing. Also, it helps you accurately guess the part you couldn't hear if you know the speaker is strictly following proper grammar rules. It doesn't fix your listening problem, but it's much better than ignoring the details you can't hear, I think.
There must be many other ways to address this. But the point is that improving comprehension alone won't get you very far when it comes to output. You should pay attention to details you might have ignored. If you don't notice when one particle is wrong when it's obvious to native speakers, it's only natural you make the same kind of error in speaking/writing. Exposure alone won't fix that if you keep ignoring details. If you're frustrated that you can't form a grammatically correct sentence quickly when your comprehension level must be good enough, maybe it's because you can't judge the grammatical correctness at the speed spontaneous speech or writing requires. If you think it takes too long to come up with the right word, maybe that's because you can't tell when the wrong but similar word is used when you read quickly or listen to dialogue at a normal speed.
So, I think the root cause of your problem is the simple and obvious fact that your speaking and writing are never better than your listening and reading. If you can't detect the discrepancy between correct and natural wording X and a slightly different and unnatural version Y when spoken to you or when you read them, it would be too much to expect to be able to form the correct form X at a natural speed in spontaneous conversation, texting, and so on. Certainly things aren't that simple, and there must be many other factors. But reading your post (and many others' posts by beginners), I kind of feel like some people just don't get the idea that your active skills (i.e., writing and speaking) can't be better than your passive skills (i.e., reading and listening).
Edited: 2012-04-05, 9:39 pm
.
let's both definately remember to report how it goes!!! If it works, perhaps it'll help others too... i'm also very interested in hearing from someone who's doing it with Japanese, because French has many more expressions that seem to map 1-1 onto english, so it's a little easier there.