ITV4 now on Freeview Light transmitters: retune needed for all Freeview homes
If you have Freeview Light
If you have full Freeview number
However, older boxes can get "confused" when a channel moves between multiplexes. This is because they don't know to "release" the 24 slot first, and finding the "new" ITV4 will stick it on 824 or the first free number between 800 and 899.
If you have this problem, the solution is to follow this:
Why ITV4 and not ITV3 (or ITVbe)
ITV3 is a popular channel (the best of ITV archive), ITV4 does contain a considerable amount of live sport (such as the Tour De France). Also ITV3 is available on the PSB3 multiplex in the Channel Islands
Why have +1 channels on the Freeview Light transmitters?
The main reason is that only PSB2 is designed for ITV advertising: if ITV+1 was on a commercial multiplex it wouldn't be able to have the correct local adverts. This is required for "Contract Rights Renewal remedy (the CRR remedy)" - http://licensing.ofcom.org.uk/tv-broadcast-licences/other-issues/contract-rights-renewal - which has been put in place because ITV channel dominates the UK television adverts market due to the exceptional number of viewers.