Can I replace 2-3way switches with 2-4way switches as I have a couple of new 4ways just sitting around and the 3way switches are the older style ( every other switch in the house is the new style)
Sure, a 4-way is essentially two 3-ways in one device. Connect travellers as usual, then connect the power or light wire to one of the remaining terminals, leaving the other empty.
Background Information (if anyone cares)
The following illustration demonstrates the internal workings of each type of switch in a conventional installation. You should be able to see how a 4-way (middle )is functionally two 3-ways (left and right switches) overlaid.