I'm going to have a 4g coral-only pico reef.

Whats the % of water i must change weekly and biweekly?

Thanks!

I would do a gallon weekly

25% (1 gallon) every one to two weeks depending on the type of corals, your feeding regimen for those corals, the filtration system you are using, and what you see in terms of nitrate et al. buildup. Shorter times between changes if you have leather corals, feed heavily, don't run any form of filtration, see your Ca and CaCO3 levels getting depleted quickly (or in the case of these dose for it) and/or see nitrates etc above 5 ppm. Also keep track of your Salinity and make sure to top off accordingly.

This one is without a skimmer.
25% is good then?

I do about 1 gallon 2x a week.