I feel that anything that can give you even a rough idea of your conditions is better than just guessing with nothing. It's sort of an emergency deal - you do what you can do right now.

You said you have an 18x4, which is likely an Intex Easy-Set. These guys are pretty close to 18 feet in diameter at the base, and taper in something like 10 inches or a foot near the top. The waterline is about 30-something inches above the floor. I measured it once, and although I don't recall the exact dimensions, I did calculate it to be right at 6000 gallons.
One gallon of 6% bleach is 0.06 gallon of actual sodium hypochlorite. In 6000 gallons, 0.06 gallons is a ratio of 6000:0.06, or 600,000:6 or 100,000:1 or 1,000,000 to 10. That's 10 parts chlorine per million parts of water. Easiest pool math you'll ever run up on.

Two gallons of bleach will put 20 ppm into the pool. That will START killing the nasties in the pool. The nasties will fight back and consume the free chlorine in the process. You will need to keep the chlorine level high to outgun and outlast the algae and bacteria and bugs and lions and tigers and bears. Oh my. This (and loss from sunlight/UV exposure) is why you need to check your chlorine levels so that you can keep it high. You could need a few gallons for the next few days.

Oh yeah... you may want a couple of filter cartridges too. Once the algae starts dying, it'll start gunking the filter. You can rinse it off with a good stiff spray from a garden hose every couple of days to keep it going, but it'll eventually clog.