My friend gave me a book full of ideas. They often require a lot of preparation by a parent, so I have adapted some of them. My kids were a little younger, but I think they'd like them.
1. Strip 'em down and put them in the bathtub with a dish tub (or similar) full of warm water. I sat one on each side of the tub. Give them scooping things, like measuring cups, plastic stuff from the recycling bin, etc. and other fun things that are different from their usual bath toys (my MIL used to give the kids a spatula with holes in it and a plastic whisk). Oh, I left my little one in a diaper to prevent potential grossness.
For a little extra fun, you can get bath fizzies that change the color of the water. My kids LOVED playing in blue or yellow or green water.
When they water's gone, you can fill it up again or say it's all done and get them out.
2. Cut up shapes out of colored paper. Wrapping paper is fun. Give the kids a piece of blank paper and a glue stick and let them glue the shapes on. The 1 yr old might need help, but this is still a sitting-down activity! After a few tries, kids can make pictures out of the shapes. My son made a tree and a train once.
3. Drawing in cornmeal. Get a rectangular cake pan and spread a layer of cornmeal in the bottom. The 4 yr old will like drawing in it with his finger, then wiping it away again. It an get a little messy, but nothing a vacuum can't handle.
4. Ask the 4 yr old if he can build a MegaBlocks tower as tall as himself, or taller.
5. Get an empty laundry basket and have the kids toss stuffed animals into it as "basketball." This game comes in handy later, when you're cleaning up. They can play "basketball" with Megablocks into the big bin they come in, or into the toybox, or whatever..
My friend sent me this link for ideas to build with Legos. They might not have Legos in the house because of the toddler, though:
http://creative.lego.com/en-us/FamilyTime/buildinginstruc...
My personal addition is to sit the kids in front of PBSKids.org for a while. They have great educational games on there that the 4 yr old can do, and the 1 yr old will probably love watching.