I have a time sheet spreadsheet for employees

i have cell c4 allocated for user to input time sheet number ,lets say 1500

when the user goes to next time sheet in workbook i would like them not to be able to input 1500 but instead only be able to input 1501 which would be the next number in sequence. if they skip a number i would like it to show an error and if they input a used number i would like it to show an error.

currently i have a drop down list in the cell c4 and that list is taken from another page which is called rates

Is it possible to have the number 1500 deleted from the drop down list after the user has already used it so that they can not use it again and only display the next number in sequence of the last number used.

thanks for any help