If that 4% held true at my lowly level, I should be capable of an 18:45 5km, as that equates to a 10km of 39:00.
I suspect away from the elites, the difference would be greater than 4% as my gut feel is my flat out 5km-and-no-more would be closer to 18:30 than 18:45. I might be overly optimistic though.
Sub 18:30 would be my guess based on your longer splits. Parkrun test beckons