Did you update the application (e.g. new hotfix) since installing PE4?
You are correct - they pulled PE4 support for 7.1 officially (I asked my friend who used to maintain it). The problem becomes for those who put it in place, and then add another hotfix to the application - at that point, the PE4 MIGHT no longer be working for them! I have no idea the response to that. In the past, every HF release, the PE4 build had to be rebuilt - by "us" in support (before I left). Dev didin't do it, support did. PM and Management said "NO MORE" to that, and so now, if you apply a later HF, not sure if there might not be code changes that make the PE4 environment not work right. I just don't know.
Out of curiosity, what's holding you back from DS 7.5? I KNOW not everyone can go there, just asking. Some don't just cause they don't want to. Some have 2 yrs of paperwork to do first. I get it. But 7.5 works with PE4 AND still gets development. More than I can say for 7.1.
Anyway, keep us posted...