What are the significant differences?

I have both.  You can go to Spyderco's website for the dimensional differences, but first and foremost, handle shape.  

The Endura 4 has kind of a long, somewhat ungainly handle for my girly hands and the PM II's shape seems a bit more conservative/traditional.  Most PM II/III have G-10 scales, where the Endura's are FRN--fiberglass re-enforced nylon.  One has a lock back locking bar/mechanism and one has a compression lock (PM II), which is thought to be stronger, but I think that might be pickin' nits.  While the Endura comes in some specialty steels, like ZDP-189 and HAP40, the PM II comes in a plethora of different steels besides the standard S30V and my PM II happens to be CPM S110v with cobalt blue G-10 scales (that's really why I got it, lol.)

The standard Endura blades are VG10 and made in Japan, the PM IIs are completely made in America.

Chris

In addition to ^^
PM2 has a finger choil, a bigger lanyard hole and I think a bigger spyder hole.

The compression lock is awesome.

Much better jimping on the pm2, & the finger choil is nice. Endura's have a longer blade but a very thin handle so they are more comfortable to carry for me. Love both knives but since the pm2 my endura's collect dust.