Help.

a rat pup is 4 weeks old?

Rat pinks - from birth to about 8 or 9 days old - eyes closed, ears flat to skull, no hair, you can however see their pattern in the skin, exist soley on mother's milk, will die without her to feed them, keep them warm and lick their butt ends so they can eliminate waste.

Rat fuzzies - from about 8 or 9 days old to around 14 or 15 days old, eyes closed, ears still flat to skull, have a fuzzy quality to their new coats (almost pelt like), may start to wiggle out of the nest to blindly follow the mother rat, still fully dependent on her milk supply though some will try her food supply as well here or there.

Rat pups - from about 15 days old until weaned, eyes are open, ears are fully upright and their full coat is now in, they will start to eat mom's food and try the water bottle, usually by day 20 they are eating and drinking this way but will continue to nurse from the mother as much as they can and she will allow (by this time most female rats are sick of their offspring LOL)

We usually wean our rat litters at about 4.5 weeks of age (about 33 to 35 days average). I give them one month plus 5 days from their date of birth, then they go into the feeder grower tubs (seperated by gender) and the mom gets a well deserved rest before re-breeding her.

As far as getting the right size, here's a handy trick. Take a shoestring, wrap it loosely around the largest girth of your new snake. Put a knot in it. Take it along to the pet shop that breeds rats. Check it against the butt/hip area of different rat sizes. You want a rat that is no larger than or better yet slightly smaller in it's largest girth than your snake. Eventually you'll be able to eyeball it, but for now the shoestring trick will help you bring home the perfect size.

Also I should have mentioned, there can be a huge difference in size in each stage. A rat pup of say 15 days of age is much smaller than one almost at weaning age (say about 25 days or thereafter). Even in the fuzzy and pink stage they are growing almost before your eyes so a few days makes for a lot of growth. Also you have to take into account the mother's supply of milk for them, their own genetics and the size of the litter. Some litters produce fuzzies that are significantly larger than another litter. Some rat young at weaning age are twice the size of others so the words pink, fuzzy, pup, weanling, etc. are just guidelines to age and average growth really.