My background in Japanese-- 6 semesters in grad school, 3 years removed. (So I had forgotten a lot.) I was around the 1000 mark in RTK1 when I went there.
I did 2 weeks at Yamasa this past fall. They did a good job of testing me to find my level of proficiency, and asked me what I wanted the most from my 2 weeks there. I emphasized speaking, since I can do reading/writing in the U.S.
I spent about 3 hours a day in class. 1 hour in a lesson (since it was just me, I always had to answer the questions

), then 2 hours in CALL. During the CALL sessions, I'd have to go up and answer some questions verbally for about 15 minutes every hour, so I spent about 1.5 hours talking, and 1.5 hours on the computer doing everything else each day. The CALL computers are a little slow, but you get used to it. Real JP keyboards, though. I waaaant one now.
Homework wasn't too bad. Some of it was stuff I already knew, but needed to review, some of it was stuff I didn't know. Nice mix, and it didn't take more than an hour or two to finish. Again, I was there more as a tourist than as a student.
I worked with 2 teachers there, and both were very nice. Very different personalities, too.
If you're going there for 4 weeks, I'd recommend going into one of the groups, and not doing it alone. While I learned a lot, I felt kind of isolated being the only person in my class. The minimum stay to get put in a group is 4 weeks, so with only 2 weeks for me, I did private lessons.
For me, and the purpose of my trip, those 2 weeks were great. It gave me a chance to brush the rust off of my Japanese, and it gave me a sort of "soft landing" in Japan. I needed the time to get over the jet lag, get used to things there, and just sort of un-disorient myself.
After I finished at Yamasa, I spent the next 3 weeks traveling all over the country. I went to: Nagoya, Kyoto, Nara, Hiroshima, Fukuoka, back up to Osaka, then Nagano, Sendai, and finally a week in Tokyo. (I went to Nagoya while at Yamasa... something to do on the weekends.)
Yes, it was awesome. And exhausting, but I got 90,000 yen worth of travel out of my 50,000 yen rail pass. But I digress.
I was comfortable enough after staying in Yamasa to not feel very stressed about traveling all over the place. Man, I love the JP rail system. I
really miss it. ;_; I had enough language tools to get things done, and I could even carry on basic conversations... so long as my hotel room didn't have any English-language TV.
Okazaki is... well... to put it nicely... dull. Rent a bicycle when you get there. It adds to the fun, because you can get to places of less dullness a little faster. And make sure you get a decent bicycle. Nagoya is 26 minutes by special/limited express, and only 610 yen to get there. It's a nice escape on the weekends.
Your main issue is probably going to be housing, due to high demand, but even in the dorms, they have A/C in each room. (Fun with kanji- learn how to decipher the remote controls for the A/C!) There are some hidden costs-- futon rental is one of them. You can either buy or rent. I rented, because it turned out to be cheaper than buying. But the futons weren't all that comfortable. They have wooden platforms you put your futon on, and then you sleep on that. Kind of brutal on my back at first, but you get used to it.
The common kitchens in the student village didn't do much for me. I'm a clean freak, I guess, and I haven't lived in a dorm since undergrad many moons ago, so my tolerance is a little low. The conbini next door has cheap food that won't kill you. (But the food at the Denny's in Okazaki will. *shudder*)
There's also a coin laundry right next to the conbini, which is great, too. The machines even have detergent built in. No need to add your own. The dorms have their own machines, but they're tiny and slow, and kind of old. They're okay as a last resort.
I'm making it sound like the dorms are bad, but they're not. It's the nature of dorms. I would have preferred an apartment, but they were booked up.
The dorms wound up costing me ~40000 yen for 2 weeks total, which was fine with me. Tuition was ~$600 US. I probably could have saved a lot more money if I had stayed longer, but it couldn't be helped. I had a limited amount of time.
And of course, ZigZag is the place to go. (On-campus bar.) Nice bar, nice food, good people, big TV, and the cheapest Guinness in Japan. It's a good place to meet up with all kinds of people-- students and locals.
Oh, and if you go, bring slippers.
EDIT: Sorry-- didn't mean to write a novel. One other thing-- you get access to the online CALL classes for a year after you go. If you're looking for sentences to mine for AJATT, this is a Good Thing.