I believe they should be merged with one as a synonym.
Both representations have widespread usage, but the Roman numerals are how it is represented on the official ULA webpage for it. The only major issue is the delta4-heavy tag already exists and would need to be renamed too for consistency.
The only other potential issue (which is pretty minor) is delta-iv implies a potential delta-v rocket, but that tag is already in use to represent the concept of delta-v (change in velocity). However, I think the time when a "Delta V" rocket would have been seriously considered is past.
Anyway, I'm down for renaming both tags, but I can see how this might be controversial, so I'm going to offer both options in separate answers.