I have the 3G model of Lumia 640 XL. I think about buying a new 4G wireless WiFi router and connect My phone to it through WiFi. My question is, will my phone benefit from the high speed of 4G router if I connected my phone to its WiFi network?
In principle, yes. The Lumia 640 supports 802.11n WiFi, which is generally faster than 3G.
In practice, the speeds you get will depend on 4G coverage in your area and your cellular plan, as well as the distance and presence of obstacles between your phone and the router, interfering signals from other WiFi networks, and several other factors.