Can I replace 3-way switches with 4-way switches?
Sure, a 4-way is essentially two 3-ways in one device. Usually a 4 way switch has 2 (IN) screws and 2 (OUT) screws. Using one of the (IN) terminals is the same as the common terminal on a 3 way. And using the (OUT) terminals is the same as the traveler terminals on a 3 way. Connect the travellers to the (OUT) terminals, then connect the power or light wire to one of the remaining (IN) 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.