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],"","-",[4,[1]])

Any suggestions?
Thanks Russ

[techexpressinc]

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

[recyan]

Hi,

Is there a possibility of having values such as below ?

AB
GATES
HAV Z

Edit : We both posted at the same time.
Thanks for sharing the solution.

Thanks

[techexpressinc]

The length of the field was 4 characters in. And there were at least 3 characters to start with. I was matching up plan-ids from two systems. One just worked with the first 3 characters. The other had a hypen and then a number i.e. ABC-001 and ABC-002, etc.