How To make an A4 sound chesty?

You can't use 'chest voice' / speech quality at that range, but you can learn to belt. Two very different things. Chesting will ultimately result in you hurting yourself.

There are potential dangers in belting incorrectly, so best to get a teacher, I'd recommend an Estill teacher, they are well trained to deliver training in this (http://www.trainmyvoice.com)./ There are others too, but probably hard to know who is good at it. Here's an article explaining some aspects of it:

http://en.m.wikipedia.org/wiki/Belting_(music)

Another answer said "belting". It's sort of a weird thing, but it's true. On a similar vein, you can resonate sounds at different pitches at once. My father, as an example, when he talks loud (he doesn't even know that he does it), has a mixture tone that has about 5 different pitches that I can recognize, the lowest and main one a bass, the highest background one above A4. He doesn't use his diaphragm to do this. His voice just carries long distances on several different frequencies. I'm not sure this is something you can learn on purpose. It's one of those "can you turn your tongue upside down" things. Some people simply can't. Training might help, but there's no guarantee. I suffer from the same problem as my father. People often tell me I don't have to talk so loud when I really am not. I'm just hitting a side frequency on top of the carrier signal that bothers their inner ear.