What?
The 4 Series is a coupe version of the 3 Series. It's not really an opinion...
Traditionally, all versions of the 3 Series (Coupe, Convertible, Estate, Saloon etc) were called 3 Series but after Audi decided to use A5 as the name for the 2 door A4 BMW responded by renaming the coupe and convertible versions of the 3 Series as 4 Series instead. The fact you think it's a completely different car when it isn't probably helps explain the reason behind the marketing
It's the same car, the difference between a 3 Series Saloon and a 4 Series Coupe is exactly the same as the difference between the previous 3 Series Saloon and 3 Series Coupe.
The 4 Series is always a year or two after the 3 Series, which means a 2019 3 Series is an all new car and a 2019 4 Series is still the 2011 3 Series. There is now a new 4 Series to match the 3 Series but it didn't appear until late 2020 and is therefore far above the budget.