If your 4G signal strength in your area is already good then any attenuation by using a more sensitive aerial won't make much of a noticeable difference.
Think of it like this. In order for the 4G base station to be able to "hear" people near and afar it tells those up close to "talk softly" and those afar to "talk loudly" so that the "volume" of each talking person arrives at the same level at the base station's "ear". If your 4G base station can already "hear" you well enough and you decide to use a "megaphone" (high gain antenna) then the protocol will force you to lower your "voice" - which can then introduce a bad signal to noise ratio.