Electrical – Combining multiple electrical runs into a single wire

Step one - determine (or tell us) if these are all one circuit (and are there garage lights other than the opener and your bench lights, and are those on the same circuit as any/all the outlets, or not?)

Step two - there are rules of thumb, though they can be adapted to actual use-cases, such as your low-draw LED lights. Following them won't keep things happy if too many heavy-duty things are all running at once. Exceeding them won't make the circuit blow if 27 outlets each have a 10 watt LED light plugged in. 8 (duplex) receptacles on a 15 amp and 10 on a 20 amp circuit is one such RoT - I've also seen 7 suggested. As such your plan to add 4 duplex would fit within nearly every RoT on a 20 amp circuit with 3 outlets now. Also, you are running the same stuff now on the same circuit without trouble, which is a good sign, until you add more stuff running at the same time, or open the garage door while running a power saw and sprinkling the lawn.

If you have a panel/sub-panel in the garage, running a new circuit is probably worthwhile - if it needs to come from the house main panel, that may be more work and expense than you want to go to for a setup that works now from extension cords.

The wires for your outlets would run from the LOAD side of the GFCI (there may already be wires connected there if it's all one circuit in the garage) and then run to the first outlet, from the first to the second, etc.

THIS IS A GUESS - This Explanation and diagrams must be confirmed with a tester.

I added this as a comment but I'll go ahead and make an answer so I can post 2 drawings. This is what I think you have going on, I think the original plan was for the circuit to have just 2 switches as classic 3-way, like this:

(Excuse my Paint abilities, NOTE: GROUNDS NOT SHOWN

The Next day, Next week or Next year someone decided to add a 3rd switch, the load end of the original 3-way circuit became the 4-way, like this: This is one explanation for what I see in your pics, but I can't see everything needed. THIS IS A GUESS AT THIS POINT, using these diagrams as a guide you need to test and confirm the accuracy.

The clues that suggest this explaination to me are the white wire attaching to the common screw in your second pic, and in the first pic a white wire nutted to a pair of black wires. The guess is that's the same white wire. A good first test would be to check for voltage where the white joins the 2 blacks, if my theory is correct that will but hot when the circuit is on and not hot when the circuit is off.