The 4ppm is just a starting point so you dont have to top it up for a while.
If you are cycling out 2ppm ammonia into 0 ammonia and nitrite in 24 hours you are considered cycled. You could introduce a moderate amount of stock in 1 go, but i would urge caution and add fish gradually. A moderately stocked wont be producing as much a 2ppm ammonia in a day, but that level of beneficial bacteria will ensure that the lower amounts of ammonia your tank is producing is gone in minutes not hours.
If you are cycling out 2ppm ammonia into 0 ammonia and nitrite in 24 hours you are considered cycled. You could introduce a moderate amount of stock in 1 go, but i would urge caution and add fish gradually. A moderately stocked wont be producing as much a 2ppm ammonia in a day, but that level of beneficial bacteria will ensure that the lower amounts of ammonia your tank is producing is gone in minutes not hours.