Or, rather than monkeying with replacing the joists entirely, just sister them. Take a board sized for however much load you're anticipating, put it up against the side of each existing 2x4 joist from support to support, and through nail it every 6 inches or so from alternate sides with 10d coated sinkers.
Bevel the top of the new board on the low end of the deck to clear the deck, just make sure it's bearing on the wall top plates so you're not trying to feed the load back through that 2x4.
To finish, take one long 2x4 (or multiple sections spliced if you need the length), laid flat on top of the new sistered boards at the middle of the span, and nail it into the top of each sistered board with two 10d nails. Your current 2x4s are under tension and didn't need the bracing, but your new boards will be under combined tension and bending, so they do need at least that little bit of bracing at the center.
You can scale this approach to hold however much weight you need, within reason. Higher loads might need more bracing, and higher loads farther apart might actually require you to sister a few extra 2x4s to wall studs underneath. A lot depends on what you're planning to hang. For what you'd normally put up in a bunch of rubbermaid bins in the garage, sistered 2x6 boards and the bracing described would probably be adequate.
Hi. I've got a one car garage that, thanks to you, I've been motivated to improve. I installed a circuit of outlets and a light circuit with a switch as there was none there before. I did this all on the right wall with the open studs. I'm in the process of covering it with peg board as you can see one 8x4 up already in the back right corner. 