If memtest shows just ONE error, you have a problem.
Rerun, testing just one stick at a time.
If you suspect the mobo ram slot, test a good stick in each of the slots.
Ram is sold in kits for good reasons. The ram vendor will want to replace the whole kit.
If you must run with just the good ram, and can't be without a PC while waiting for a replacement, be prepared to reason with them. They may accomodate you.
First I congratulate you on knowing to run memtest. Many skip this important test.
I agree with geo - do the testing with single RAMs as he suggests.
After that, if you have trouble with any single stick or still have trouble with all four, then I suggest you check further before calling vendor for replacement.
What is the full make and model of the memory you have?
Is it on the list of compatible memory for that mobo?
Alternatively, is that mobo on the memory manufacturers list for that mobo?
Do the timing settings of your installed memory match the recommended list?
I have the same mobo but not the same CPU or memory. When I build my PC, it ran any one chip fine, or any two in duo mode, but when I went to 3 or 4 I got errors. I had to do a lot of futzing with the timings and other BIOS settings but finally got it to run solid.