Big growth spurt between week 3 and 4, they more than doubled in volume. It is, in other words, "stretching." Still feeding Bio-Grow only (for now) and watching for signs of flowering, when the plan is to add Top-Max and Bio-Bloom.
There's a faint smell now in the tent, nothing much and it gets filtered fully.
Day 27 (end of week 4 therefore) and the larger of the plants starts forming pistils (see the close-up photo). Looks like everything's on track by entering the flowering stage come week 5.