excel+vba+training - If Statement - Excel

Hello

I have this IF statement which should work out the commission for a customer baed on how many units they sell
Here is the breakdown of the commission factors (they get a percentage of a set fee)

0-500 35%
501-750 40%
751-1000 45%
1001+ 50%

Now i have the formula for the IF statement as follows, but it doesnt work, can you tell me where im going wrong please

=IF(E4>=1001,((B4*50%)*(E4-1001))+((B4*45%)*250))+((B4*40%)*250)+((B4*35%)*500),IF(E4>751,((B4*45%)*(E4-751))+((B4*40%)*250))+((B4*35%)*500)),IF((E4>501,(B4*40%)*(E4-501))+((B4*35%)*500)),IF(E4<500,(B4*35%)*501),0)))