Not all rats are equal !
It all depends on the breeder and how their raised(primarly fed)
One batch 3-4" weigh more then some in another batch 4-5"
Older ? nope ! Just better fed.
I would go with 4.5" for a snake that size,if he`s hungry he`ll eat it. After two feeds he should smash a 5"r
Our Big`Un is right at 37" and pounds 5.5"-6`rs like nothing.
Bottomline,
Always use a reliable source for feeders.