I’ve only been playing for several weeks, and I’ve gotten several 4* crew that way. Eventually I stopped doing the 10-at-a-time thing, though, because as a free-to-play player, I was running out of slots. So I decided to wait until I was up to 250,000 and use that instead to purchase 5 new crew slots. 
But as G O R N says, it’s not all that frequent—you’ll end up with a lot more 2* and 1* crew than the rarer ones. But that can be good too, if you’re able to fuse an existing character. Alas, I keep getting freebies of 2* crew that I’ve already fused. Just today I had to dump a 2* Sisko for that reason, and I know I’ve spaced several 2* Picards, as well—as much as it pains me.