Interpretation of a clause from inherited lease

[Jefferson A.]

Hello BP! This is my first time posting. I'm having a problem understanding a clause on my inherited lease. I need help with the interpretation of this clause;

"If tenants fail to pay any portion of the rent or any other payment require under this lease within fifteen days after it shall be due, tenants agree to and shall pay an additional sum of four percent (4%) of the amount not timely paid for the first month (retroactive to the first day of the month) and for each month or portion thereof for which said rent payment is not paid, as a late charge..."

but then there's this next clause right after the one above;

"If tenants fail to promptly pay rental payment and are delinquent for a period of five days after the payment due date, Landlord may serve Notice to Quit on the fifth day..."

So my understanding is I charge a late fee on the 15th and serve the notice to quit on the 5th? Hoping you could help me. TIA