Ok, I wanted to post an update. After burning the 4th backup without success, I thought I'd check into lens issues due to the fact that it played fine using an emulator. I have other backups that play fine on the playstation, but with THPS4 I had no luck.
So at
http://www.geocities.com/psxrepair/ I found a tutorial for lens cleaning. I completed step 1 (cleaning the lens with a cotton swap dipped in lighter fluid), but that didn't help any. Although I didn't think it would do any good I went ahead and completed step two, taking the playstation lens cover unit apart, removing all grease, cleaning the parts with lighter fluid, then greasing them again. I skipped the lens power adjusting part.
To my total surprise my backup now runs on the playstation!
Thanks for reading!
Marie