I think it would be fine to swim in it if you like.

That said, that test result of 3-4CC indicates a DESPERATE need to shock your pool and I mean DESPERATE.

Because your CYA is so high, you have probably never reached shock level in your pool for a long time. You need to bring your chlorine level up to 30+ (with bleach only) and keep it there for a while.

I'd start in the evening with 12 gallons (yep) of 6% clorox. Test it again the next morning and add enough to get back up to 30...it'll probably have dropped pretty significantly. Repeat this procedure each day until your test the following morning shows very little loss of chlorine (less than 3 or so).

Then, let your Cl drift down into a 8-15 range. You should keep it in that range the rest of the Summer.

Read the sticky's posted at the top of each forum...you'll gain a wealth of information.