It's normal to forget some kanji as they advance through the stacks...The point of the system is that the website takes care of all the scheduling details, so try not to worry about it.  (Unless you fail a kanji repeatedly, in which case you probably need to rework you story)

And if your retention rate starts to drop  (people have different theories on this, but I'll toss out below 80%), you probably need to reduce the number of new cards.  Sounds like you're doing 80 or 90 cards a day, which is quite a lot.  It's doable, but as time goes on, you will have a lot of reviews to keep up with.

I'm not sure what you're saying, are you saying you're forgetting your stories but not the kanji? If so, while it's ULTIMATELY not a problem to forget the stories (they're just tools), it's a problem to do so this early. I wouldn't fail the cards, just make sure to go through the story over again in your mind when you review them. This is because the story will probably stick in your memory, uhm, middle-term better than the kanji will. Since you've only been doing it for 4 days, there's still a lot of short term memory working in your ability to simply recall a kanji without the mnemonic, so, again, reinforce the stories when you drill the kanji so that you can hopefully continue to remember them later.

I went through the first few chapters very quickly, using very few stories. I just remembered most of the kanji on sight. However, once I got further along, I ran into a lot of problems with interfernce: i.e. I started learning new kanji that were similar to the earlier ones, and I began to get the two confused. I had to go back and relearn the previous ones with better stories in order to differentiate between similar ones.

This is why 切 has the highest failure rate in my deck despite its simplicity. It quickly made its way into the fourth stack, but then once I was around 500 kanji, I suddenly started confusing it with other kanji every time I saw it!

The moral of the, erm, story is... spend time making good stories now even when the kanji seem very easy. Things only get harder as you go along, so you want to lay a good foundation.

Trust me, rely on the stories. If you don't remember the stories, make new better ones. You might think you know the kanji well enough, but what you're doing is wasting all that the imaginative memory style gives you. I studied kanji for a year in Japan and I learned almost 800 kanji... a few months later, while I could still read them, I couldn't write most of them. Was it because I never learned them well enough? No, it was because I knew many similar kanji and I remembered how they looked but made tiny errors. An extra stroke here, misplaced radical there... The stories make sure you write the kanji 100% correct, all the time. Eventually, there will be kanji where you do not remember the stories (epecially radicals since they are usually hard to make good stories for) and that's not really a problem but try to keep that to a minimum. It will help you down the line.