You always lose a little quality whenever you transcode from one codec to another, video or audio, but perhaps you can avoid it if you only want to change from MP4 container type to an AVI container.
codec:copy may be helpful if you don't have a reason to transcode (and it should be faster).
If you must transcode because you need to use a particular video codec
I hope you can use zetah's suggestion above in his comment and use the
-sameq switch. Alternatively, you may want to use the
qscale option to set the quality manually. The higher the number the better the quality, but the more space your file will take.
The last time I transcoded video I used
qscale=8.0, but I'd suggest that you experiment to find the optimum to match the quality of your input.
Marty Jay helpfully mentions that sameq means same quantizer as in the input, which may not result in the same quality. The article he quotes mentions using multi-pass conversion, which is a good way to achieve better compression without sacrificing quality.