K.P.
Puzzles, books, library cards, play-doh "sets", play jewelry (dollar store), art supplies (Toys R Us has a great sale on Crayola right now), dolls, tickets to a children's activity, membership to a local play gym or museum...
If your mom lives locally, she could buy a set of holiday cookie cutters and have the 4 yr old spend a morning baking with her... purchase the apron and the cutters (great for play-doh and stencils too) that she can take with her at the end of the day. Then she could wrap the goodies to give to the family as a gift! My mom did this with my son last spring for Easter and they had a blast!