Hello. I've got back into cisco after a long while and I have completely forgotten what I am doing on it haha. The problem here is I can't get the PCs to ping each other, unless they are connected to the same switch. I'm not sure what I'm doing wrong...

Hi there, I'm a university student and I'm trying to get a head start on Cisco so I'm not too big on it yet. When I try to ping PC0 and PC1 they work just fine, however when I try to ping PC2 or PC3 with PC0 or PC1, I get the destination host unreach...