Big Agnes' Insulated REM Insulated Air Core pads ($70 in regular length; www.bigagnes.com) are interesting creations. New on the market, they're air-filled mattresses containing Primaloft, which adds insulation while still allowing the comfort of a true air mattress (not air and foam). Big Agnes uses temperature ratings rather than R-value, so it's a little tough to compare to the ProLite, but the Air Core is rated for three-season use and down to about 15 degrees Fahrenheit. So on the face of things, probably not quite as warm as the ProLite 4. But more comfortable. And very lightonly one pound, four ounces.
InsulMat, your other consideration, also doesn't list an R-value for their Max Thermo ($54; www.rei.com). This pad, like the Air Core, has insulation inside the air-filled tubes, but it too is rated as a three-season pad.
Bottom line: I think you're still stuck with the closed-cell pad. But, you could pack any of these pads, still save some weight, and probably add some comfort!