The way you're doing it is fine and safe. I wouldn't do it any faster. The Zoas will probably adapt to the high salinity fairly quickly, it's not at a lethal level so don't panic.
I'm guessing that your water temp is somewhere around 80F. You didn't provide a Salinity but if you're around 80F then the Salinity is around 42ppt. If you keep your tank warmer, above 85F or so, then you are in the danger zone at 44ppt but you still shouldn't panic. Just keep doing the 4 gallon changes and it will happen fast enough.