Let's say you enter dates in A2 and down.
In B2, enter =IF(A2="","",B2+28) and fill down as far as you want.
With the range in column B still selected (and B2 the active cell in the selection):
On the Home tab of the ribbon, click Conditional Formatting > New Rule...
Select 'Use a formula to determine which cells to format'.
Enter the formula
Activate the Fill tab.
Click OK, then click OK again.