Acrylic is generally cheaper and lighter, but won't last as long as zirconia and will not look as realistic. So, if money is not the driving force in the decision, then Zirconia is the way to go. "Lifetime" is a tough expectation to put on the longevity of any dentistry because materials aren't indestructible. It also depends on the bite that the all-on-4 is set to. If it's a proper, neuromuscular bite, then there's less stress on the arch, but there are also things like your eating habits, proper care of the dentures and implants, regular hygiene appointments, etc that will all affect how long the dentures last. Zirconia ultimately will last longer because it's a stronger material. Also, depending on the restoration such as a denture or bridge, even though the technique is called 'All-On-4', you will likely need more than 4 implants if you opt for Zirconia because it's a heavier material and will require more support.