I had this on my mark 3.. Fortunately it was pretty simple to get off and was right on the outside of the mirror or something. It'll be really small so youre going to need a lot of light and to look very carefully. just open it up, put it under a strong lamp or something and then carefully use a q-tip or a rolled up piece of lens cleaning paper or somethin very sterile and particle free to get it out once you see it. i tried a rocket blower but it wouldnt budge it and then i just touched it with a rolled up piece of optical paper and it came right off.
yours might be a different problem though, maybe its just on the outside of the viewfinder? it could be deeper in but thats unlikely since itd be pretty hard for something to go in that far, so my guess would be that its somewhere on the outer parts of the inside, I dont know the technical terms for all the pieces.