Help with nested Excel formula returning "FALSE" result

Can someone tell me how to fix the following formula? I get "False" result if the value in cell B5 is greater than 999. Thanks!

=IF(B5<251,(B5*12)),IF(B5<1001,3000+(B5-250)*8.75),IF(B5>1001,B5<4999,9562.5+(B5-1000*4.25)),IF(B5>5000,26562.5+(B5-5000)*2.88)