You know your snake and you know what it's been eating up to this point - if she's been eating small rats up til now, I'm sure she's fine. For some places, a small rat is really a rat hopper or rat crawler, which can be about the size of a large mouse. She may also be older than 4 months, especially if the breeder was feeding her small rats before you got her.

I've had babies get large enough to eat small rats in their first 4 to 6 months, so it's not entirely unheard of. I also have the option to get very small small rats, so it's easy to bump them up early on.

Snakes can often sense things we can't, and if she didn't want to eat the rat, then she didn't want to eat it. I wouldn't worry too much, just perhaps try offering that rat to the male next week, and offer her a new one instead. It happens sometimes. I have a little long tailed grass lizard I got over a year ago to feed my Indonesian Tree Boas, as they hadn't switched to rodents yet. The boas wouldn't eat her. I figured maybe they didn't like that kind of lizard, and brought home some anoles and a male grass lizard to pair her up with. The boas ate the anoles AND the new male grass lizard, but they didn't touch her. Fast forward 6 months, and I ended up giving the boas to a coworker who is working with them, but I STILL have this female grass lizard that the boas wouldn't eat. I put the grass lizard in with my blue tongues, figuring up until she was eaten, she'd at least enjoy the space.
A week goes by, and she is basking on their heads. Fine, I say, my frilleds will eat her. They've eaten house geckos before when I put them in there for stray cricket control.
No. No they don't. They eat the second male grass lizard I get to pair up with her, and all the house geckos I try keeping in there. But not this one female long tailed grass lizard, who is now fat and sassy and all over that cage. I've given up at this point, she's totally healthy and fine but apparently nothing wants to eat her. She must smell funny or something. I don't know. But now she's got a name (Ingrid the Indestructible) and she's hanging around.

So, long story short, maybe the rat is sick, or maybe your snake can smell or sense something about it that we can't. It's not like our brains work the same as a snake's does, so it's impossible to say for sure. Good luck with the next feeding!

-Jen