Your question is uncliear. If you are trying to enter months which are
spaced four mounths apart then
Enter Jan in A1 and Apr in A2, highlight both and drag the fill handle down.
If you are trying to increment a date not just the month label then enter
the first date in A1 and =EDATE(A1,4) in cell A2 and copy the formula down.
EDATE requires the attachment of the Analysis ToolPak. You can also enter a
start date such as 1/15/09 in A1 and 4/15/09 in A2, highlight both and drag
the fill handle down.