They need to do a lot at DAK and DHS, as well as more work at EPCOT, before they think about building a 5th park.
EPCOT doesn't really need expansions, especially compared to the other two, but it does need to fix several existing spaces. A new World Showcase pavilion or two and a couple of additional rides up there would be great, but not really necessary if they didn't have some old Future World pavilions wasting away.
DAK and DHS, on the other hand, both need at least double the amount of their current attractions.