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.

There should be no problem doing what you want, as long as all the conductors and overcurrent devices are sized properly. As per National Electrical Code, you'll size the equipment grounding conductor (EGC) based on the largest overcurrent device used. Which means the EGC will likely be sized based on the size of the breaker protecting the 240 volt circuit.

National Electrical Code 2014

Chapter 2 Wiring and Protection

Article 250 Grounding and Bonding

250.122 Size of Equipment Grounding Conductors.

(C) Multiple Circuits. Where a single equipment grounding conductor is run with multiple circuits in the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray. Equipment grounding conductors installed in cable trays shall meet the minimum requirements of 392.10(B)(l)(c).