You can, but only with the Year 3 materials!
We strongly recommend that you teach the material in sequence because each part makes the next part much more understandable. Numerous words that are explicitly taught and practised in Year 3 are taken for granted in the Year 4 lessons.
Many Year 4, especially those with weaker literacy, would be stumbling over a lot of vocabulary if they hadn’t done the Year 3 work. This would slow progress and limit enjoyment. You will be startled by how future Year 4s will fly with their reading and all the Year 4 tasks once they have done the Year 3 programme.
If you would like to get next year’s Year 4 a little bit ahead, however, you can start teaching them (as this year’s Year 3) during the summer term. We recommend that Year 3 teachers do this anyway, because it is a good way to practise teaching with the materials.
Current Year 3 teachers can therefore teach Year 3 Autumn 1 and Autumn 2 during this summer term, i.e. starting straight after Easter. This will also mean that Year 3 teachers and SLT get even more out Training Half-Day 2, because you will arrive at the training with some experience of actually teaching the programme.