I've been lucky in being able to split utilities (water) or already having things separately metered.
Do you happen to have bills with one unit empty (so that you could potentially look at usage of just one unit)?
I would consider splitting the bill by squarefootage. If the 4/1 is 1200sqft and the 1/1 is 400sqft, then billing back 75% (1200 out of 1600 sqft total) to the 4/1 and 25% (400 out of 1600 total) to the 1/1.
You could also alter this slightly by adding a portion that you cover (maybe you pay for 20%, then have the other tenants pay for 60% and 20%) to help eliminate "They're using way more than they're paying" or whatever other complaints may come.
As a side note, I know there can be some issues with a landlord "acting as a utility company". It's something I've heard on a few podcasts. It might be worth looking into for your area/situation. I don't know that I've heard of an issue with a landlord charging back utilities on a couple of units.
Obviously, having separate meters would be the most direct way to go, but that may not be possible.