heres a few, but some are what you mentioned already
http://www.freewarepocketpc.net/ppc-tag-youtube.html but i dont know than any change the file version from flv to mp4 though like you want... also whats wrong with using the stock youtube app in the phone? is there a greater purpose you have in mind to using them as .mp4?? or the need to be physically saved VS just bookmarking them in the apps? just wondering