4G emphasizes range and mobility over rate - and costs more. Your phone will work 5km from a basestation, or in a train at 200kmh. You can get (say) 150Mbps from a basestation shared between hundreds of users over dozens of km2. That basestation will cost a lot (perhaps $100,000 including electronics, tower & site).
WiFi emphasizes speed over short distances. You can get 400Mbps (say) from a new WiFi AP but only over a few tens of metres and only for a few users before QoS matters.
So having both is a good thing.
The two do converge with carrier WiFi and small cells. So, for example, at a shopping mall or airport it would make sense to integrate both (and 3G) into one box: you serve more people, and the cost, hassle is per node (installatioin, electrics, backhaul) so Return-on-hassle is improved.
But at home... I am an enthusiast of small cells and I am not convinced of the need for LTE femtocells at home over WiFi.