Good question, Gina. The hibiscus you received have been fertilized every time they are watered, which is almost every day because it is so hot in the greenhouse. If you stop fertilizing them now they will not be very happy. For best results, use the fertilizer once a week or even more often if you can. They will grow better and bloom more and the flowers will be more colorful.
The 4 inch pots should not be planted straight into the ground. But, you say you have the 6 inch pots so it is safe to plant them in the ground any time now. You will find when you gently pull the pot off the plant that they already have roots that have grown to the bottom of the pot. It might be a help to read the page called "Potting and Planting Hibiscus" on the HVH web site at this link:
http://www.hiddenvalleyhibiscus.com/care/potting.htmAnybody else have any tips about planting straight into your garden?
Charlie