Only way to achive 4+ effciency is with CASUS. But they are too much error prone. There i propose create new item called neutron reflector. If this item is added next to uranium cell it will act like another uranium cell but it will not produce another heat or power. For example if we place one uranium cell and one neutron reflector next to each other it will produce 20EU/t and 20 heat/s. If you place one uranium cell and surround it with four reflectors it will have 50EU/t and 50 heat/s.
real world counterpart http://en.wikipedia.org/wiki/Neutron_reflector
A neutron reflector is any material that reflects neutrons. The material may be graphite, beryllium, steel , and tungsten carbide, or other materials.
So I propose recipe for neutron reflector will be from carbon plate, refined iron and/or advanced alloy.