I understand how Grimm's law has resulted in pairs such as duo / two, tri / three, penta / five. But how do we get "four"? I looked it up in the dictionary and the IE root is kwetwer- Why doesn't our number four start with /h/ ?
Frederik Kortlandt in Labials, velars and labiovelars in Germanic. North-Western European Language Evolution 30 (1997), 45-50. addresses how in many cases PIE *kw > p > Germanic f in words such as 'wolf', 'eleven', 'twelve', 'leave', 'liver' and a similar change (gw > b > p) in 'warp'. However he still thinks 'four' gains it's first consonant from similarity to 'five'