XPS 8940, ram upgrade 4x8GB?

Yes, your proposed total of 32 GB RAM (four x 8 GB) should work, though it is not a configuration that Dell tested and approved. Buy from a vendor like Crucial that allows returns/refunds if it doesn't work.

Here is Crucial RAM guaranteed compatible with your system. You can also buy RAM from Dell that is guaranteed compatible with your 8940.

Be aware that sometimes mismatched RAM brands/models may not play well together, which is why it's often recommended to use matched pairs of identical RAM modules. If the specs match, though, it may well work, and the Crucial RAM is guaranteed so you can return it if necessary.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.)