Man, this is the same story as every shortcut I've ever tried to take in life. You think you can shave a few hours off the wait because you're excited and it looks fine on the surface, and then the whole thing falls apart in front of you. The recipe said 8 hours for a reason, and that reason is physics, not the recipe writer being bossy. People act like waiting is dead time, but waiting is usually the actual work. Your cake needed time to get cold all the way through so the glaze would grab onto it, and 4 hours only chills the outside. Same thing happens with paint, concrete, and hair dye. You rush the set time and you pay for it later. Buttercream is a solid call for now.

Cheap stuff saved me $200 on headers and they're still fine after 3 years.

Got a Canadian quarter once, dude just shrugged and walked out.