1) I may not be able to convince partner that I have as little high card strength as I do. I would not want a slow auction on this hand.
2) I do not want the opps to be able to find a minor fit: this could even be a double-game swing hand (altho possession of 2 aces suggests not)
I am not overly concerned that ♠ will be the wrong trump suit. Partner would need at least 5♥ for ♥ to be a desirable suit, unless he has a very strong hand with short ♠. It is far more likely that my possession of a hidden Axxx suit will be a benefit to me in the play, especially against weak opps, who may not be counting very well
And I am not worried about missing slam here: I have about the playing strength for my bid.
Plus, as has been noted, 4♠ may catch the opps in a pressure position, while 1♠ will allow them room to realize that the 5-level is too dangerous.
But this is close: add either major J and I'd be 50-50 to open 1♠.
Not much of a line here, but there is always a line between two bids, and I know where it falls for me on this one.

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