R4 launched on 30 September 1967 as a replacement for the BBC Home Service.
The BBC Longwave (running from Daventry in 1926) closed during WWII.
Before R4 took the channel it was BBC Light Program (now R2) on 200 kHz, but moved to 198 kHz as part of a general frequency reorganisation in Europe.
See
https://www.bbc.co.uk/reception/work-warning/news/radio4lw
Like all AM closures the overall saving is insignificant compared to salaries of the better known presenters, never mind upper management and TV generally.
It’s a political soft target and the BBC seems to think streaming is the future, but it’s complementary to broadcast.