A-life is totally doable in UE4 - A-life systems are in no way dependent on having large terrains. STALKER’s terrains are actually comparatively pretty small to a lot of UE4 environments. There are one or two really cool interviews out there with STALKER’s AI programmer, but creating an A-life system comes down to two things - first is a goal-oriented AI system that models various needs and objectives, such as hunger, thirst, health, current mission etc, and how to fulfil these goals.
The other part is a system to simulate AI behaviour outside of the loaded level. STALKER achieves this by basically placing actors not in the current level into a turn-based behaviour simulation that runs in the background and keeps track of an actor’s rough geographical location and status. Both of these systems are tough to get going in any engine unless you are a dedicated coder, but it’s totally possible to do.