I played 7.0 mixed seniors and 18+, so I have seen both.
Actually, I don't disagree with you except on one thing.
IME, people who are at the top of their level tend to try to play up rather than down when they have the choice. For this reason, guys who are strong 4.0s (bordering on 4.5) don't like to play 7.0 mixed. To them, it is kinda lame. They prefer 8.0 mixed. 7.0 mixed tends to attract the lower half of 4.0 men (not that there's anything wrong with that!).
My experience at 8.0 mixed bears this out. When I played 7.0 mixed as a 3.5 woman, I could hang with the 4.0 man well enough. I could return their serves, more or less. They tended not to take the net, and their volleys could be a bit dodgy. They were not bad players, mind you. Not at all.
When I got to 8.0 mixed, however, I got to see the upper end of 4.0 mixed.
I grant that your league could be different. So I agree that OP should ask around a bit and see just how many upper end 4.0 guys are playing 7.0 mixed. I'm thinking it won't be that many.
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