WiFi is actually faster than 4G. Well, let me rephrase that. WiFi on a broadband connection (cable or better) will be fast than 4G, as cable internet has way faster latency/ping times than 4G does. Sure, the throughput might be higher on 4G, but its all about how long its takes to send and receive that request for a connection and the data. High through put just allows for more to be downloaded at the same time, but if you don't have a quick response time, then its just not quick. Make sense?
Do a speed test on WiFi and then on 4G. Which ever has the lowest ping is usually what's going to be fast, unless the throughput on WiFi is 2-4Mbps or slower.
My WiFi is almost always below 15ms in latency, where as my 4G is always around 75~100ms. My WiFi is ~15Mbps (with several other PC's and cell phones) and 4G is usually 35~40Mbps. My WiFi browses and downloads faster. And probably will until you gets down to about 4~5Mbps is when you will probably notice a difference between the two.
If your WiFi is on DSL or anything else, then 4G it is.
I never use 4G. I am always within range of WiFi that is faster, so I turn off my 4G to 3G and just leave WiFi on, and it helps with my battery as well.
SO, it all just depends on what WiFi you have.