Yes they will. All weBoost signal boosters, regardless whether they are 4G or 3G models, are engineered to be backward compatible with earlier generation networks.
If you have a 4G booster, it will boost the 4G signal when 4G service is available. If 4G signal is NOT available, or if you are using a 3G service (like voice currently on most North American mobile networks) then the booster will boost the 3G signal.
If you were to ever find yourself in a situation where neither 4G nor 3G service were available, the booster would still boost signal on an available 2G network.