Bridge: what would 3S mean here?

Question by e-mail from my most regular partner:

My response to him:

Thoughts? Either about what 3S should mean, or the best way to ask about it at

My gut instincts.

I agree that 4S would imply strong rebiddable spades, but also be limited, saying 4S is our contract. 3S should be forcing with at least a 5-card spade suit. Double is forcing with more balanced support.