I don't want to wade through the edit trail, which is extensive, but it really looks to me like you're slowly adding the info needed to provide an answer, and it's frustrating a user. The user seems a bit grumpy, but certainly not over the top.
My bent is to simply ignore such questions, or close vote as "unclear what you're asking"
The first frustration, for me, is the long edit trail. It makes it difficult to understand the history of the question. There is no formal policy on this, but if you've gone through three edits and still need to change something, you might consider deleting and starting fresh (or ask a mod to help with deleting). The long edit trail is a pretty clear sign that there is a problem.
As near as I can figure out, your main question is "How fast a signal can a motherboard handle?" without even providing information on which motherboard. The user's issue with this is pretty valid. That makes the question underspecced. For all I know, you're talking about a PDP/8. Do you mean "all computers a person is likely to be able to buy today?" Do phones count? Chromebooks? How fast is the motherboard clock?? I suspect that will make a difference.
Then, it looks like orbiting around the main question is "how can I program this"? When the whole question has an anchor that's floating around, that can get pretty frustrating to answer.
You also really never get around to saying what "combining four fans" means. Are you trying to drive them all with one PWM signal? Are you trying to just AND their Hall Effect outputs together? Are you building a fan control board? Without details like this, this is an XY problem.
The best approach for this particular question, for a start at least, would be a question on SU to the effect of "is there a standard for a motherboard fan interface, and where can I find it?" I think that question would float on this stack, too.
FWIW, a quick google for 4 wire fan control standard yielded https://folk.uio.no/kyrrens/diverse/viftekontroller/developer-specs-REV1_2_Public.pdf as a first hit.