Is there any way to convert flash videos to mp4 on mac?

There most certainly is. MPEG Streamclip is free, can convert all your FLV (or any other transport stream) files, WMV, or other video to pretty much any other format and frame size, can do it in batch mode, and runs on Macs and Windows. In fact, I set all my flash files to automatically open in MPEG Streamclip because it's easy to view, trim, and output them from the one application.

Squared 5 - MPEG Streamclip video converter for Mac OS X