Mass Spectroscopy peak assignments in but-3-en-1-ol
As you have pointed out, the peak at M = 73 is M+H, protonation is however more likely to occur at the oxygen atom. 



In my opinion, peaks at M = 66 and 67, i.e. with a rather small mass difference to $\mathrm{M^+}$, are negligible.

Common sense and the link in permeakra's comment suggest that the major fragmentation of but-3-en-1-ol occurs along the red dotted line, yielding an allylic and a hydroxymethyl fragment.