Yeah, but then we'd have to say onety, twoty, threety and fivety as well. Just take it as its own word rather than four+ty and it'll make more sense, I think.
honestly we should all just switch to chinese writing, it makes so much more sense to simply have characters for concepts than to have symbols that don't even properly correspond to sounds..
Logo-phonetic script (Chinese) is a bitch to learn and impossible to type. There were efforts to come up with a native Chinese keyboard and they all failed. The just use an English keyboard and spell phonetically through a conversion layer. Plus, alphabets, abjads, and syllabaries make it possible to recognize a word you've never seen before. I'm not an expert in this area though, I just know some words most people don't.