typically some of these "barriers"--and i have it in quotes because we place some magical value on round numbers, like sub-5, or sub-4, or sub-10--have as large a mental component as physical. while a road mile wouldn't count for your overall goal, the change in scenery might give you confidence, and distract you from the particulars of your goal, and allow you to just run. a sub-5 on the road, even with a downhill advantage, may give you some insights into the overall effort involved, or more importantly, it may de-mystify the effort.
good luck with your next effort. for what it's worth, my recommendation would be to listen maybe for the first 200m or 400m split and then throw away the watch and just compete...
cush