Q on Waste Pipe Size (3" / 4"?)

Hello - I have a fairly simple (or dumb) question about the waste pipe size that toilets use in the USA (and if your really competent, I would like info on the UK). My house is from around 77-79'ish (if that helps) and I am confused on the standard toilet waste pipe size. I am looking at bringing a toilet from Europe back to the states [screw local codes... tho it is still 1.6gpf, ill take it out when i leave ] (output via S pipe on bottom) but I think most toilets are 4" in UK... my concern is that If my pipe were to be 3" i would be out-of luck for a novice DIY-guy. Toilet facility has never been moved in house-history. If someone could help me before i dig the toilet up. would only like to battle with it once (if i get the UK toilet). Any positive information is greatly appreciated. Thank You

size

Whether it is 3" or 4" is immaterial. The floor flange that the toilet fastens to is a standard size. The only difference is the size of the outlet hole in the flange.

U.K fixture

As the previous poster said, the U.S. flange is the same for both sizes. The REAL question is what is the size/mounting method of U.K. toilets. I know european toilets are sold here, but are they redesigned for the US market so they fit?