I'd bid 4♥. I think 1) It is certainly possible that LHO has four spades. I'd choose to preempt on a lot of hands that are 4-5 with bad spades. But I agree it is fair to assume that partner likely has exactly 3 spades. 2) Limiting partner to 8 points is sort of pessimistic. If you double aggressively, partner may choose to bid only 3♥ on hands which are up to 10 points, expecting you to raise to 4♥ with hands like this. 3) Club/heart finesses, if needed, are likely on. That said, I don't think this is particularly obvious, and I could be easily convinced that the probability of losing 3 spades + another trick is too high.