I do pretty much everything with MPEG Streamclip
But for cropping video to 3:4 screen it is not the most easy.
Have to calculate it by this calculation:
unscaled (wide pix) - 4:3 (wide pix) : 2 = X
Then enable cropping and put X to Left and Right.
If you also need to scale it because you still end up with a to big video. Select Other
fill in 640x480 and at cropping setting select Source instead of destination.
Always preview the result before you make the export, to check it. I also use the cropping to remove any black bars.
I'm used to select Apple PhotoJPEG, but that's another discussion.
This is also a good option or ffmpegX iFFmpeg
There are many more, but this might give a start.