In STEP 4 children are not taught any new phonemes or graphemes. Instead, they are taught to further manipulate the phonemes and graphemes they have already learnt.
Many of the words children explored in STEP 2 and 3 were monosyllabic (words of one syllable).
In STEP 4 children explore more polysyllabic words (words containing more than one syllable).
Many of the words in STEP 2 and 3 required children to blend approximately three sounds together in order to read them.
STEP 4 requires children to blend an increasing number of sounds together in order to read.