She has probably moved the nest. There are a lot of differing opinions about how and when to "rescue" feral kittens. Living in a rural area, we have the luxury of leaving them with mom until they're 8 - 10 weeks old before we try to trap them. Unless mom is trapped pregnant, we never recommend people look for the nest, because if you don't get them all, mom will move it. At this stage, we just let mom bring the kittens to us.
I believe the best thing to do is put food out for about two hours - either in the morning or the evening (or both). Whatever you decide to do, be consistent and regular.
If they were 4 weeks old, they're about the age she'd start to bring them with her to eat.
If you live in an area where they're relatively safe living outside, I'd let them be with mom for another 4 - 5 weeks. Ideally kittens are with mom for 12 weeks (they learn how to eat, clean, manage their pee and poop so they don't have litter box issues later when they're using a litter box, etc.), but they're much easier to adopt out when they're younger - and the less time left outside, the less danger to them.
When they're used to the food being left out at the same time and in the same place every day, and they've been with mom for 8 or 9 weeks, then it's time to trap them.
You can either purchase a trap, call around to local vets to see if they have a trap you can borrow, or you can call around to local shelters or rescues to see if they have a trap you can borrow. You can also see if they've got space to take the kittens, if they provide free or low-cost spay services, or know of any place that does. This way you can trap mom too and have her spayed and return her.
If mom is feral and not a stray, she would be difficult to socialize, and most rescues would kill her rather than have someone foster her. If she's spayed and returned to where she was trapped, at least she can live her life without providing more homeless cats.
Here are some links that will help you find shelters, rescue groups, TNR groups - all which may be able to help with information re: fostering/adoption, vet services, spay/neuter services, etc.http://www.pets911.comhttp://www.alleycat.org/orgs.html
trapping and other info: http://www.thecatsite.com/forums/showthread.php?t=16045
...and there's more links and info available at the sites you can click on in my signature line.
I'm so sorry the kittens were given back to the mom rather than taken to a vet.
Good luck rescuing them a second time around,