Given that Aunty Beeb will still have to fork out for the production costs (or the costs of buying in programming from external providers), I'm surprised that they'll save that much through not having to pay Arqiva and Astra to use their transmitters and satellites for transmitting BBC4.
However, given that the majority of TV viewers now have access to BBC iPlayer directly on their TV sets (rather than on their computers, etc), through smart TVs or services which give 'smart' capability to non-smart sets (such as Amazon Fire TV Sticks or Roku devices), it might not actually make a great deal of difference anyway.
I occasionally use BBC iPlayer (via my NOW TV stick) to watch BBC Scotland and BBC Alba, which obviously aren't available here in East Anglia via terrestrial TV, so it won't really be any different to me to access BBC4 in the same way.