Ok, differences ..
Xfwm4 window manager, so has Win7 style windows snapping, transparency, drop shadows, rounded window corners, built in compositor, etc.
(making it much more configurable, customisable, and better looking .. but not any heavier that I can tell)
New icon theme
New version of PCManFM, ICE, and a few other doodads
More cutting edge software available because ti's based on Ubuntu/Lubuntu 13.04 .. eg. LibreOffice 4 is in the default repos.
Got rid of GWOffice (which was broken in PM3) and replaced with Google drive
Besides the Xfwm4 window manager and the fact it's on the 13.04 code base there are no MAJOR differences .. it just looks/behaves a whole lot
slicker .. so no need to rush into an upgrade .. your call
It's mainly about lots of little tweaks and the newer code base .. but then it was never going to have any major changes (besides the Window manager) .. how do you improve on perfection
It's now IMHO the most "polished" and "professional" looking LXDE distro available.. but you'll have to decide that for yourself .. as I said, nothing major, just improvements .. if you liked PM3, you'll like PM4 but just that bit more.
Have a looksee in a LiveCD/LiveUSB and decide for yourself.