What Mini 10 do you have? The 10v/1011 and 1012 take standard DDR2 laptop memory. It should work in just about any DDR2 laptop with a free memory slot. The 1018 takes DDR3. Same situation.
The 1010 is largely unupgradeable. Any one who sold you memory for the 1010 is probably mistaken or lying.
So, the question again is "which Mini do you have?". Or do you know that you have a DDR2 laptop module? A quick Google suggests that the XPS M1210 takes up to two DDR2 modules, each up to 2GB.
Be aware that you might not see the full 4GB, I have a similarly old Intel laptop, which is limited to roughly 3.3GB, even with a 64bit OS.