According to one source, the BB1000 is for a 73mm (mountain bike) bottom bracket shell width. Quite probably, it will also fit a 68mm shell with spacers, but check first. While the 19mm spindle diameter works for either, FSA says the BB1000 is "compatible with Gamma Drive cranks," and the BB4000 is "only compatible with FSA Omega MegaExo road chainsets."

The two are pretty much the same price (Black Friday notwithstanding), so it might be more reassuring to just replace your BB4000 with another one.

If you decide to replace the bearings instead, they're FSA MR087, and they fit either of those BB units. I've never replaced the bearings on an external BB, but you might need some kind of bearing press.