Can anyone identify this deficiency? 4th week of f...

Not a deficiency, this is the start of burning from over feeding and/or a salt build up from over feeding. In soil, a schedule of plain water/plain water/fertilizer works well, but plain water/weak fertilizer/plain water/weak fertilizer can work if you are careful. In soil, you should not be feeding any more often than every 7-10 days, and certainly not every two days. Obviously, if growing in coco coir, you need to feed every irrigation (with run-off), however, a weaker solution may be better than using full strength every time.

Ca deficiency does not look like that. Mg deficiency takes 4-6 weeks from inception of the physical lacking to the point you can see symptoms. These two molecules are unique and their leaf symptoms are drastically different. Could be too much p or k, this often happens due to the popular belief that you need to blast it with p and k in flower. As usual, it's not about more but the 'right' amount. Too much S can burn the serated tips too. Low K is possible, too, but you'd probably see some interveinal chlorosis coinciding with the burnt tips around outside of leaf fingers. Ther's not enough info to eliminate possibilities. Diagnosing based on leaft symptoms alone is a poor way to do it. LEaf symptoms are not discrete. Not only do many symptoms overlap, but whether or not it is a phyisical lacking or overdose vs a nutrinet being locked out by other things in too high of concentration is mostly impossible to discern by just looking at the plant. Consider recent behaviours and look over any progression over time coinciding with how you have fertilized and that can help eliminate possibilities. Tracking how and when you add things can help you revise and do better next time, too. Fertilization is the culmination of everything you've done since day one. IT's not just about what you did last night or last week. How you fed early on can drasticaly impact needs later, which makes one garden potentially significantly different than the next as far as doing what others do and not putting deeper thought into it.

I would say environment issues here dark green shinny leaves all dried out and crispy looking, you will have more of an idea to what's going on if you have measurements of everything they should tell you. Keep a diary

NEED A WELL DOCUMENTED DIARY FOR PROPER HELP!!! YOU SAY IT IS ON DA NEWEST LEAVES!!! DAT CAN TELL YOU A FEW THINGS!!! IT WONT BE POTASSIUM AS POTASSIUM IS MOBILE!!! DAT ISSUE WILL FIRST SHOW UP ON DA OLDER LEAVES!!! ENVIRONMENT,PH IMBALANCE COULD BE ISSUES BUT ONLY A GUESS!! LACK OF REAL INFO HERE!!!

Looks like the beginning of some potassium, but plats are looking lush over all. Maybe a buildup or something preventing the update of it. Need a diary for better understanding of what you are feeding.