So they have reduced the number of KS2 classes (and hence number of teachers) by 1 via this route?
2x Y3
1x Y4, 1x Y4/5
1xY5
1x Y5/6 1x Y6
I would sound a note of caution. Almost all the other posters are describing schools with a stable, established mixed age group class format, with teachers who are used to teaching this way and thus in-school experience about how best to manage it.
Suddenly creating mixed-year classes within a school that has never run them is a VERY different situation.
Equally, a situation where e.g. all Y4s are in the same class as all Y3s (as happens in many small schools) is different from a setup in which some Y5s are with Y4s and some are with Y6s.
Ideally, every year 5 should have the same experience and curriculum - in terms of topics, books read in English, subject coverage, trips, visitors etc. Schools with established mixed classes have longer term planning that ensures this, for example with a multiple-year planning cycle which means that every child covers e.g. the Romans once and once only, but some will do it in Y4 and some in Y3.
This is absolutely ESSENTIAL so that the Y5s in with Y6 don't repeat topics in Y6, and the Y5s in Y4 don't miss any material that they would have been taught in the pure Y5 class, so they are ALL fully prepared for Y6 with no gaps.
With a last-minute decision like this, when are the 5 teachers affected (the three 'pure' Y4,5 and 6 teachers and the two with mixed year group classes) going to sit down and align their curricula and practice, so that all Y6s have the same experience / topics, all Y4s have the same experience / topics and, critically, all Y5s have the same experience / topics?
IME established, mixed classes in which every child of the two or more year groups affected is included in the mix, with an established rolling coverage of topics and teachers who are experienced in teaching this way work very well. Those where only some pupils are in a mixed class, with inexperienced teachers, work much less well. Adding in the timing, no established plans for rolling topics, and no in-house experience of working this way seems to me to be a recipe for a disaster for this particular year group and the staff. I anticipate that staff turnover and absence this year will be VERY high.