Let's assume that the battery in the pager is replaced with half C4 half battery. AA battery weighs about 20-25g, so 12.5g C4 total. 12.5g of C4 has roughly 84,000J of energy. For comparison, a 62 grain 5.56 NATO round has about 1,700 J of kinetic energy. The C4 is undirected, so the target is getting a fraction of the impact, but that much C4 detonating near your body is going to do some serious damage.