I’m probably not the best person to speak on this as I’ve never really used UE3. However, it is more than likely now being less supported than UE4, and UE4 will probably give a lot more possibilites going forward? All the documentation and resources are on the forums and answer Hub here as easy access to any issues you come across.
Also, while UE4 does have some amazing graphics and can really push high end systems, you are able to drop your settings right down for your game for them to run on the lower spec machines.