I don't think that you can do Thor in four letters and be "period", as both Thor and Tor makes for three letters in the Futhark. Thorr, though, is still correct, and that is a four-letter word ('specially if you're a Christian). The ON were not so particular with their spelling, though, so if you want to write T-H-O-R rather than TH-O-R, you certainly can. All I'm saying is that I have never seen it done that way in the old sources.