It should work. The matched pairs thing is more of a best practices thing these days, it's not really required like it used to be. It does impact dual channel memory operation, and you will most likely lose out on that particular feature if you go with a 1x4GB 1x1GB setup, but I would expect that the performance boost from the extra RAM would more than compensate now that the system doesn't have to hit the HDD as often. Dual and even triple channel memory have long been solutions in desperate search of a problem anyway. On a Mac Pro you might notice the performance difference, but I doubt you'd ever be able to tell on a Mini.
I'm too lazy to look up what the max amount of RAM for the 2010 Mini is, but suffice it to say that one of three things will happen:
1: It works exactly like you hope, you see 5GB of RAM when you go to About This Mac
2: That 1GB module is ignored, and you only have 4GB of RAM
3: The system will not boot until you remove one of the modules
Fortunately the new Minis have a very easy to access design, unlike all previous models that required some careful putty knife work. If you run into situation #3, it's pretty quick and easy to open it back up and remove one of the modules.