When your month evaluates to 6, you're trying to create a date with a month value of 0 (which is not valid).
This formula can also have issues if the date you're adding 4.5 years to is on the 29th, 30th, or 31st of August (which would put you at February 30th, for example)...or several other dates (like October 31st).
Probably the easiest/most reliable way you can do this with a formula is just to add the number of days to the date. Adding over 4 years means you will encounter a leap day (except for years like 2100 which are divisible by 4, but not 400), so the number of days you want to add is
(365 * 4) + 1 + (365 / 2) = 1643.5. Round to whatever integer you see fit.
Your formula would end up being
<Date field api name> + 1644