Speedplay shoes/cleats

4 bolt may lower the stack height a little, and allow for slightly easier mounting, if that matters to you, but the range of compatible shoes is limited.

Northwave do a speedplay specific adapter for their three bolt shoes, which gives a very low stack height and is pretty easy to fit, lots of adjustability both fore/aft and lateral. If you can get Northwave shoes on offer then it's definiately worth it, and still keeps the ability to go back to three bolt cleats if you want.. 

I've used X pedals for years, for a long time with 3-bolt (Sidi), but for the last four years with 4-bolt (Lake). With the 4-bolt you can ditch the adapter and save a some weight, but that's about it.  If the shoes you like are three bolt I wouldn't worry too much unless you are after every gram.  For what it's worth my 4 bolt Lakes were no more expensive than the 3-bolt version of the same shoe, so have a look around.

The pedals are great either way!

I think the range of 4 bolt shoes is both limited and expensive. They're probably marginally easier to deal with but I'm not sure it's worth it. I'm very happy with Speedplays on my cheapo 3 bolt shoes. 

I've been using speedplay zero pedals and cleats with 3 bolt shoes (first Shimano, now Fizik) for about a year. Can't see how using 4 bolt shoes would make them any better. Very limited range of 4 bolt shoes when I last looked.