One of the things Ada used to say was “when I’m four”; this designated a point in time when, in her mind, she might stretch her abilities beyond a certain point. She would, for example, count to 120 (in English) or 20 (in French) and, in spite of being quite capable of going further, explain, “not now, when I am four”.
Rosa has never vocalised the same idea, but having turned four, she suddenly seems to have decided to switch on a bunch of things: an interest in learning to read, colouring in the lines, writing her name, and, in the last week, a dramatic increase in the complexity of her sentences - previously, she had good but terse use of the language, getting her point across in a fairly direct fashion. As though she is obeying some unwritted rule she has suddenly started producing more elaborate, beautiful sentences; the amount of information in them is no different, but they are far more decorative.