First grow and im on week 4 day 3 of flowering

They are smaller but that could be a number of things.
One could be genetics
Lights
Feed
Environmental

let us know what the conditions are and what strain she is

Purple stems do indicate a phosphorus def.
Thr middle number on the npk
When switching from veg to flower, I continue veg feed as I feed preflower, using a P ratio that’s at the 45-50 range. I do this till week 4 (strain dependent) then I go full flower/bloom feed with a slightly lower middle number and a higher K value.
let us know what you have been feeding