Is it correct to say "It's a 4 hour stopover."?
Yes. That's okay, but . . .
But to be correct, you should put a hyphen between "4" and "hour":
It's a 4-hour stopover.
And it's more formal to use the word "four":
It's a four-hour stopover.
The hyphen is because both "four" and "hour" are working together to modify "stopover".
What kind of stopover is it? A four-hour stopover.
The same is true whenever a two-word combination functions as a single adjective to modify a noun:
She's a well-behaved child. (What kind of child?)
He was hell-bent on getting it done. (What was he?)