This is partly caused by the strange used car market and partly caused by the odd product life cycle of the 4 Series.
Remember, despite being literally a hatchback 3 Series it was 2 years between the launch of the 3 Series and the launch of the 4 Series Gran Coupe. This means that at the other end of the products life, you get this situation whereby the 4 Series continues in production for a further 2 years alongside the brand new thoroughly modern miles better 3 Series (Because BMW always try to get 7 years out of a platform). This means that even before the weird car market problems people buying late model 4 Series were paying big money for an old car - the current situation has only added it to.