3mm vs 5mm 4D acrylic
Both thicknesses use the same laser-cut acrylic and the same character shape. The difference is entirely in how far the character stands off the plate, and that has knock-on effects worth knowing before you commit stock.
How they look
At 3mm the character sits close to the plate and reads as a crisp raised block. It is the subtler of the two.
At 5mm the sidewall is the dominant feature, throwing a deeper shadow. It is the thickness customers usually mean when they ask for a bold 4D look.
Fitting and handling
5mm characters are heavier and less forgiving of an uneven blank. Take more care on the first placement.
Thicker characters are also more exposed to knocks in a wash, so they suit a customer who will look after the plate.
Stocking both
3mm and 5mm are separate items with separate codes, so switching thickness on a page does not move quantities between them.
Most workshops hold 3mm as the default and 5mm in the characters they know sell for show work.