I wouldn't worry as much about values to the right of the decimal, it should round in. It's the ones to the left that'll cause the error. Somehow or another you've got a value of 100+. I'm confused why the > and < failed, but I'll leave that up to you unless you want to post code and errors.
Personally, I'd break it off the main query and find the problem data rows *first*, so it'd just be a
select col from table where CAST( col as NUMERIC(18,4))>= 100
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