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.