Wear-plate steels, such as Hardox, are standard practice. There once was a source of 3.2mm thick Hardox but I believe that is rapidly drying up, if not already gone. 4mm is quite easy to source, though. I'm not a heavy builder, but it seems the standard method is to design a heavy, get the armour and chassis parts watercut by a company of your choice, then weld them all together.

There are other wear steels, but I'll let others comment about those, if there are any Hardox competitors.