The requirements for changing tyres on a four-wheel-drive vehicle can vary depending on how the four-wheel-drive system is arranged. The Toyota RAV4 of this generation uses a power split transmission, so there is not a mechanical link between the front and rear wheels which could cause problems if there is a significant difference in tyre circumference. Unless the owner's manual states that all four tyres should be replaced you should not experience any issues if you just replace tyres on the same axle, although for peace of mind you may wish to replace all four. If the tyres are worn at the outside edge more than the inside this may indicate an issue with alignment, specifically the camber angle, so we would advise an alignment check when the new tyres are fitted.