Formula problem

[Rod1979]

I am looking for a way to make my formula calculate after 28 days.
It for rental rates I have it up to 28 days but if I go past that the cell refers to False.
What I want to calculate is as followed"
The rates are day/week/month with a 4 day week and 3 week month
Can any one help?

=IF($O21<4,$Q21*$O21,IF($O21<7,R21,IF($O21<11,+R21+(($O21-7)*Q21),IF($O21<14,$R21*2,IF($O21<18,($R21*2)+(($O21-14)*$Q21),$S21)))))