A friend is letting me have his amplifier to use at church. It is a Fender 1000 Roc-Pro
head with a Fender cabinet that holds 4 x 12 in speakers. He says it is not ideal for
a bass but I can trade the head for a bass head to improve sound. I guess my main
question is if the cab with the 12 inch speakers is good as-is for bass. Are some 12 inch
speakers ok for bass and some not?
It should be fine but resist the temptation to turn up to 11. The differences between amps and cabinets come down to things like what frequencies they emphasise and how they perform as they are turned up louder - a guitar amp is likely to be expected to start to distort at loud volumes where a bass amp might aim to keep things cleaner. If you're down on the bottom half of the volume dials (and I'd be surprised if you need to turn up even that far in most church settings) you should be fine.
What else is in your set up?
One other question is whether the cab is sealed or not - if the back is open then there's a fair chance you might pop a speaker or 2 through over-excursion, even at low volumes.
But as Wulf said, it's not an 'ideal' rig for bass, since all guitar stuff is designed to strongly colour tone, while most bass gear is relatively transparent.
I did end up trading the Fender Roc-pro Head for a Fender Bassman 400 Head. I got $200 trade value.
It is compatible with the speaker cabinet and sounds awesome. Most powerful thing I ever played through
ever! Might not ever need it at church... but its nice to dream :)