If you buy the 4 inch .45 then you will have to buy a 5 inch 9mm whenever one turns up.
You can look at that as settling for second best and then having to get what you really wanted to start with. And having to figure out what to do with the .45.
Or you can look at it as ending up with two different variants of an already scarce gun.
I have a few facts and a lot of opinions.