The fact is that you don't need to know how many spades partner has, this is partners job! You just need to return a small ♣, partner knows if he can cash a second spade before he returns a club.
I now see that this works if you play 3rd-5th ...
The only reason to bid something other than 4♠ should be to try for a slam, because in 4♠ LHO most of the times will pass since to go to the 5th level without any help from partner VUL is something with very low probability. Now slam has ...
I will pass. Our side will have on average about 25 HCP, if East is a decent player will not lead a spade and even if we manage to make 3-4 ♣ tricks we still need two more coming, at least 1 spade trick other than the Ace and probably ...
Partner can have ♠ AJ10xxx a K and a Q (or a K) either ♠ AKxxxx and a K or ♠ AKJxxxx and a Q.
In the last case 4♠ is a more or less hopeless contract except if the Q is in ♦, 3NT on the other hand ...