This will cover the function of the S-4 section.
It is the logistics section.
And yes, amateurs talk about tactics, and professionals talk about logistics.
It's a simple job: Get everything you need. Then get everyone, and everything they need, from point A, where it is now, to Point B, where you need it to be.
As Bill Roberts noted in a post on WRSA, the military breaks that "everything" down into ten classes of supply:
Class I – Food, rations, and water
Class II – Clothing
Class III – Petroleum, oils, and lubricants
Class IV – Fortification and barrier materials
Class V – Ammunition
Class VI – Personal Items
Class VII – Major End Items (computers, cars, etc.)
Class VIII – Medical supplies
Class IX – Repair Parts
Class X – Miscellaneous supplies
Everything falls into one of the above categories.
All the S-4 section has to do, is make sure they have all of everything required.
And sufficient means of moving it, and the unit, from where it is, to where the leadership wants/needs it.
Whether that means a train of semi-trailers, a camel caravan, or by pushing heavily-laden bicycles down a narrow trail, that's what they have to acquire, and organize.
Your S-4 is WalMart, Target, Home Depot, Costco, FedEx and UPS, all rolled into one.
Everybody may contribute to what they do, but at least one person has to be in charge of accumulating the stuff, accounting for it, and moving it.
What made Lexington and Concord a victory wasn't the operation of harassing the British troops all the way back to Boston, fun and cheerful though that aspect was; it was the twin coups of superior intelligence from their version of an S-2, and the superior logistical expertise of their version of the S-4 in moving all their muskets, cannon, shot, and powder out and away from where the British could get their hands on it, with bare notice of their imminent arrival. That success enabled them to later challenge the British for over a year, until the other 12 colonies decided to get aboard the revolution bus.
D-Day wasn't nearly so much about getting 100,000 men ashore at Normandy (brutally and primally brave though that was on Omaha Beach), it was about being able to get enough bullets, bandaids, beans, and all the other seven classes of supply ashore, day in and day out, to enable them to amass the wherewithal to break out and sweep across France a month later.
Napoleon sagely observed that "An army marches on its stomach."
Or as it was put on more than a few military S-4 shops and motor pool signs, "You may be the pride of the division, but without us, the pride don't ride." Or eat, or shoot, or anything.
What's in a person's pack will get them through a day, maybe three. But not 2 weeks.
You have to have a handle on what you're going to need, first for bare subsistence, and second, to be able to conduct any sort of training or operations. Or else, like Japanese soldiery on a bypassed island, you're just going to wither and die out in nowhere, for nothing.
The logistics section is what keeps that from happening.