First off, Yamaha hasn't released anything yet for Ampeg. Second, the 4 Pro's early problems were back in the 90s under SLM, who was two owners ago. I don't have one but I've used plenty of newer 4 Pros on gigs and never had a problem. And all my newer stuff made by LOUD still works.
It seems to me that SLM really had shifty quality control. Most of my Ampeg stuff is early 90s, SLM built, and it is at best a little iffy. My early SVT2-pro especially. Lots of *** were they thinking when opening that up.
That said, sounds good when it works.
Thou shall not talk about the quality of CRATE amps. Though again, that said - I have a 20w Crate combo with replaced MOSFET and RadioShack 10" that sounds pretty decent and is surprising flexible for what it is - which is an early 90s SLM 20w cheap ass bass combo amp. Low bar.