Even if the following books sucked after the original trilogy finished, that's the beauty of book-to-movie adaptations.
You can change things. He did so with the first movies, he'll do so again. Four is probably enough, though.
I think that's the problem with book movies. If there are many in a series, people end up wanting to do them all. Thankfully Damon only wants to work if Greengrass does it, and Greengrass hopefully has enough sense to know when to quit. Then again, four is a weird number. Ultimatum ended perfectly, but now it's open again...so he can either just end the movies with an open end, or make a fifth and risk overkill, but tie it up (Although there are six or more books).
I dunno. I think this'll be Damon's last, and therefore the last Bourne.
-AC