Just have a simple, maybe stupid question..
I have updated all the debs to 1.5.1 and have my system running very smoothly and stable. Is it important to do the entire upgarde to R4V4? What would I be gaining/missing?

if you've already done the mythtv upgrade and everything is working good then you probably wouldn't gain anything. The only reason for someone who has upgraded mythtv to upgrade to R4V4 would be if they had a software encoder and were getting hit with the bttv bug.