Yes, it is always a good idea to have all the same ram. Not just manufacture, I like to say manufacture, speed, layout type, and even Batch. Nothing stagnates in the high tech world. The ram you buy today, might not use the same modules or controller of the ram you buy next year.
If at all possible get the closest thing to it you can. Idealy get it all at one time, get it in a "set" if possible (its easier to return :-) And get one of The exact model they listed in the "motherboard compatability chart" for your board.
There is hope though, many boards are pickey when the ram chip ammount is increased, and there are even special adjustments for how it changes Only because the ammount that is there changes. Like Scew or is it Squew?? Some boards the voltage regulation isnt perfect , so changing the voltage up a bit might help. The slight differences in "timing" things can be tweaked around a bit.
I could not get Post even, with a ram combination I used on default settings. But I was a persistant litte cuss , and I knew how pickey my board could be. I went back to the 2 sticks, then way underclocked the ram. Using the ram Divider (666 800 1000 1600 or 1:1 5:4 4:3) Then tweaked the timings so they had longer "wait states" T-ras cas and whatever to higher numbers. Then I put the rest of it in . After I got some slow combination working, I Wrote it Down.
Then I just tweaked it back till it failed again, writing down each change I made. Within a few hours I had it up to 99% of where it was originally, and everything working perfect. It was just like a 9 dial combination lock :-)
So it might be possible, and 4 sticks gets pickey for a lot of people. Many people just concider the ram to be bad or incompatable, and take it back. Saving themselves a lot of trouble. Some of us are too thick to give up so easily :-)