I will add to this:
Two of the replays that failed with 4px wide were Wimpy 7 and Medi 13 (LPI). I'm fairly sure these two levels actually have identical replays (being the 2nd and 3rd versions of No Salvation), so if one fails, it's expected that the other does too.
Also, Fierce 15 (LPIII) failed on both tests, albeit in different ways (3px failed very close to the start, while 4px failed very close to the end).
Indeed, overall, it seems like the difference in replay breaking is negligable. One thing that does come to mind - the 4px variant requires more precise placement when trying to place a blocker with lemmings falling from above it (eg. under a trapdoor). This was the reason for the Mild 16 failure. It also makes it more likely that a blocker assigned in a crowd of closer-together-than-RR99 lemmings won't let any slip past; this can have both positive and negative effects (for example, them not slipping by is the cause of Wimpy 2's replay failing; but it is also something that people often find irritating from what I've seen when lemmings do slip past).