-GSXR400 cams work for the B4 and you'll get more top end power.
-The jet kits are designed for replacing main needles and such in your current carbs, just make sure they're for your carbs exactly.
-Ehh, about the slip-on, it depends, i don't know much about it, but as long as the diameter is the same about 2" (?=50-52mm?) you can fit it, but what the results will be is always kind of a surprise (more midrange or toprange?)
-Changing the front sprocket to something bigger will lower your acceleration but increase you topspeed. This is always a trade-off: "What would you specifically wish to have?"
Hope this gives you a clearer idea of what you want to do.
Also, check out the many many threads on this board, there's much info around here about nearly everything!