Hi,
if the shield has a large heat sink it should be no problem as the L298 has 2 outputs that can be used together to get 4A.
If there is only a small or no heat sink at all there is still the L298s internal overheat protection which will prevent if from failing immediately - however you won't be able to get full power and the chip may be killed after some time. (I found the L298 to be quite sensitive to constant overtemperature - despite its protection circuit)
Christian