A major consideration, when considering a sleeping pad purchase, is its temperature rating; the seasonal use for which it is designed. Some manufactures will provide a true temperature rating while others refer to an R-value and that R-value references a pads ability to retain warmth, specifically the warmth of your body during the night. More simply, the higher an R-Value the warmer a sleeping pad will be and the warmer you’ll stay throughout the night.
The materials, the construction and design of a sleeping bag, or pad, directly impact that R-value. An inflatable, un-insulated sleeping pad is designed for summer use and probably has an R-value somewhere in the range of 1.0 maybe even 0.7, a lower number and that’s fine for summer and warm weather usage. A 4-season pad with foam insulation perhaps has an R-value as high as 9.0 or even 10.0, which you’ll need if you’re looking for winter usage. Somewhere in the range of 3.5 to 4.0 is probably worth considering if you’re looking to have a true 4-season pad.
If you’ve got a 3-season pad that you’re fond of and prefer to sleep on year round you may want to consider combining it with something like this very low profile, lightweight closed cell foam pad, which will go a long way to increasing your R-value and keeping you warm enough for 4-seasonal cold weather usage.