The power your fosgates are getting would be split in half by the addition of another pair, so the sound output may decrease after adding two more subs.
Ok, I'm assuming you've got your two 4-ohm subs wired parallel, so they're a shared 2-ohm load, yes? Now, if your amp is 1-ohm stable, and I'd have to know the actual amp model to know that for sure, you could wire 4 4-ohm subs in parallel and have a 1-ohm load, which would offset the decrease in power caused by splitting the amp four ways instead of the current two. That would give you more output. Again, that's only assuming you have a 1-ohm stable amp, do NOT try that without knowing what your amp can handle. If your amp can't handle a 1-ohm load, you could wire the 4 woofers in series, and you'd have a 4-ohm load which would decrease your power even more.
If you message me or post the model of your amp I can give you more specific advice.