After you do the 4 rows, the inc row is row 5 and in the center of row 5 do the lace pattern row 5. Then you continue the pattern with a row 6 and in the center of row 6 work the lace pattern row 6. The pattern continues this way. The point is that there are border sts and increases at the sides of the shawl, and up the middle is the repeating 10 rows of the lace pattern.
I wouldn't worry too much about the decreasing part of the pattern until you get there. It's one of those things that you figure out when you have the sts on the needles. The dec row will juss be a continuation of the lace pattern with garter edge sts, stockinette sts and some k3tog decreases.