I'm not a bonsai expert, but it looks like it might be a ficus.
If it was only 4 leaves that dropped this morning, then it was already in quite bad shape.
Ficus is noted for dropping leaves when moved or stressed, but in a healthy tree, dropping a few leaves should leave it plenty.
Yours may or may not make it. You need to figure out what went wrong. To start with, the layer of rocks on top, makes air and water circulation difficult and makes it harder for you to know when it is the right time to water. They should be removed.
Tell us more about how you have been treating it - amount of sun, how, how often, how much you water, any fertilizers, etc.