Tapering four weeks before a marathon?

Hi, I am looking at a training plan for my marathon and have booked a 20 mile race. I thought it was three weeks before my marathon and it fitted  nicely into my plan However I have noticed it is four weeks before. I am looking at Hal Higdens novice plan.Would it be ok to swap and just taper with 14, 12 and 8 or should I stiick with the plan of 20, 12 and 8 and just not do the 20 mile race? Any advice would be really appreciated as don't want to lose my fitness ...help! Thanks