The reason that your chlorine isn't staying in the pool is that 1) with no CYA, the sun is "eating it up" almost as fast as you're adding it, and 2) if FC=0 and total chlorine is greater than 1, then the rest of the chlorine is being used up fighting something in the water.

Do not add any more CYA. 4 lbs in an 18000 gallon pool is more than enough, but it takes several days before it registers in the water, which is why most people overshoot their target and then regret it later. How did you add it? Did you broadcast into the pool, or is it in a sock in front of a return? Be patient until it starts to register in the water, but I wouldn't bother to test for it for at least 3-4 more days. However, if you put it in through the skimmer BEFORE you removed and soaked your cartridges, then it has all washed off and you'll need to start over.

You do need to immediately get some Borax in there and raise the pH. Anything below 7.0 is too acidic for a pool and can damage it. Start with about 1/2 box of plain 20-mule team Borax sold in the green box in the laundry aisle at WalMart (not the washing powder), and either pre-dissolve it into some water and put it in your skimmer, or slowly add it through the skimmer, breaking up the clumps. Give it a couple of hours to register, then re-test and add more as needed until you get to the 7.2-7.6 range.

In the meantime, each quart of 6% bleach will raise your chlorine by 1 ppm, and 4 1/2 gallons will take it up to shock level--you need to keep it at all times between 1-3 ppm until your CYA registers, by adding frequently throughout the day. Just a hint--if you'll get it up to shock level after dusk, when the sun is off the pool, it will have the whole night to work on "crud" in your water without interference from the sunlight, and will go a long way toward killing your impending algae bloom (usually what's happening when your water goes cloudy).

Janet