Advice needed about Marathon training and Race timing

I am following the Hal Higdon Intermediate training program for my half marathon. There are several weeks built into the schedule to do shorter races. I have opportunity to do a 4 mile race the week before my half, however on the schedule I should be doing 12 miles. But I could do the 12 miles the week before that since I am currently one week ahead of schedule. Or alternately I could do the 12 miles the day after the 4 mile race which would be 6 days before the half. So what I want to know is does it matter if I do the 12 miles one week ahead of the half or two? Will it make a difference to my performance at the half. I am new to all of this so I don't know how important following the schedule to the letter is. All advice appreciated!