The 4 and 5 pin plugs/sockets are not compatible, ie. they don't fit together. If your workshop has 5 pin sockets you will need to fit a 5 pin plug. You might find that your welder is only connected to 2 phases and earth, this is fine, it means the welder runs on a higher voltage, obtained by useing the potential between two phases, rather than between a phase and neutral (single phase).
Having 5 pin sockets is useful, it means that you can plug in all sorts of kit, some kit uses the neutral to split single phase out of the supply for control circuits etc.