If 3D is anywhere near being anywhere close to being on your radar, then I wouldn't buy a replacement projector at this time unless there was a pressing need. Will you see an upgrade with 1080p? Yes, you typically will see an upgrade. Will it be phenomenal? Not typically, though the Panasonic Smooth Screen technology does completely eliminate the screen door effect which can make the image look sharper (or softer) depending on your point of view. I'm pretty happy with what the AE4000 does with SDE and I think Panasonic really has it figured out.
The Epson 8500UB would be my choice over the 4000 because I have no use for the extra bells and whistles that the 4000 delivers, but both are really good.
But, with 3D models on the way if you can wait 12-18 months you likely will have some 1080p home theater projectors available which can handle 3D at 100+ inches and come in about the same price as the AE4000 currently does. IMO, it is worth waiting for if the price is the same.
On the other hand, if you must have a new lamp right now, then it may be time to just bite the bullet and do an upgrade.