We don't really have a basis to say that there can't be a chance, though it would be low if it exists at all, of some slowness in recovery of natural T production if choosing to go 18 days instead of 14.
However, with any type of steroid use at all I have never known of genuinely slow, problematic recovery from only (for example) 3 weeks of a short acting non-nandrolone compound.
So if you want to do the 18 days, that would be fine.
There probably is just as much benefit from a given 3 or 4 days of use whether those 3 or 4 days are tacked onto a given cycle, or whether they stand alone in a later usage.
In other words, you can go either way and be fine and should have essentially the same end outcome.