What difference does the current share price make? Why spend money to try to prop it up now with a share buyback? The only relevance the share price has is if they need to raise cash, which your premise assumes that they don’t.
Keep in mind that the "4+ year runway's worth of cash" is based upon AVXL's current spending level and that runway could certainly increase or decrease depending upon what events take place over the next 6-9 months...
Anavex has a 4+ year runway's worth of cash, but does it really need that many years in the coffers?