For take-apart fun (reverse engineering), a bicycle is a traditional project and a 4-year old would need lots of help with this... a great opportunity to talk through the names and functions of different tools and bike components.

Small wind-up music boxes are also fascinating, and the gears are somewhat similar to those found in clocks (which another poster mentioned upthread as a great item to take apart).

Kits and models may also be fun... to build and to deconstruct.

This resource also shows "how things work", although even the easier articles may be a reach for most 4-year-olds.