C4 is relatively easy to make with access to the right chemicals. The base of it is an explosive called RDX that when mixed with a plasticizer like beeswax becomes C4.

It involves dissolving hexane fuel tablets (for camping stoves) in white fuming nitric acid (somewhat harder to source, but any decent lab has this). You would want to do this in an icebath to avoid setting it off by the heat of the reaction.

Disclaimer: I obviously am not recommending trying this at home, but maybe in a zombie apocalypse or ww3