Four weeks is old enough to start giving them treats. You can just tear up the lettuce into small pieces or even leave it whole if you have a solid (or shaving) floor booder. I chop what I give them because I have a wire floor and if it's tiny pieces, I can sprinkle it on the feed trough and they don't lose too much by dropping thru the wire mesh floor. They will eat it- eventually. Chicks can be weird about new foods. If they are used to eating out of your hand, you can offer it to them that way at first. You should get one or two curious ones who will peck at it, and as soon as they appear to like it, the others will join in! I have also hung a small bunch of greens to the side of the brooder (again, it is wire) with a cable tie. Works great! They will need grit of some sort once you start feeding greens, friuts and other treats. I use sand, but a bit of dirt from the yard or the chick grit you can buy at the feed store will work fine.
I grind the seeds in the electric coffee grinder we use for spices, but a mortar and pestle would work as well. Tho at 4 weeks, they can probably handle the seeds whole.
Grass and dirt would be fine. They will have great fun tearing it apart!