Armor stacking was permitted looong long before 4e. Only two layers were allowed, the second layer was at half, FFBA and its variants stacked fully but certain things didn't work with them (i.e. no, hosehead, you cannot wear your 'underwear that's safe (and fun!) to wear' with your security or military armor), and any armor over a certain point inflicted agility and movement penalties.
You've always been able to turn a troll into a frickin' MBT. Here's the thing: that's what they're for. But doing so - armoring anyone up that much - becomes howlingly obvious. The first fight might go your way, but there are 'social modifiers', and I'm not talking about the champaigne and cocktails set, I'm talking about how people respond. Barrens? You walk through a wasteland, or you suffer constant small-arms harrassment as people try to pick you off in order to loot your Very Expensive Armor. Cops? Heaven forbid a Lone Star / Knight Errant patrol sees you, because the HTR team is gonna be on it's way, and remember that these are the guys who perfected the art of turning Tanked-Up Trolls into 3m pillars of home heating.
You can load up the armor; you can IRL, you can in the game. Can you get through it? Sure, by some method, eventually. Might take an anti-tank round, but it'll get through. Are you virtually invulnerable until then? If you can take the lickin' and keep on tickin', yeah. And it provides a challenge for the GM.
And IMO, any GM who counterattacks this sort of behavior head-on is in for a headache, because he's going to wind up with a TPK. Shadowrun isn't about combat; it's about precisely-applied pressure. That pressure can be violence, whether actual, threatened, or implied; it can be social, informational, and all sorts of other stuff. And that pressure can be - IMO should be - applied in both directions. Armored up? Watch your Notoriety go up by one or two or three for each and every run during which you stomp around all kitted up like that. You'll very soon have a reputation as nothing more than a clumsy, high-tech thug, and your own teammates are going to have to disassociate with you because they aren't, and their reputations are taking the hit just being seen with you ...