Replace Function ? - how to replace the a 4th position null with a dash

[techexpressinc]

I have a field that I need to replace the 4 character in with a dash when the position is null.
Sample of input values:
ABC
GATE
HAV
ROAD
Output values wanted:
ABC-
GATE
HAV-
ROAD

I have tried this and variations without success:
Outputfield: Replace([Inputfield],"","-")
Any suggestions?
Thanks Russ

 

[techexpressinc]

Solved
CompanyID4: IIf(Len([CompanyID4L])=3,[CompanyID4L] & "-",[CompanyID4L])