75g/90g would be my minimum with it being 4 foot long. Avoid cubes. More peaceful fish like the butterfly need to go in first with aggressive fish like the Tang last. Smaller fish are much easier fillers to add. Blennies, gobies, wrasse, bassets, etc.
Some people would like to see them in a 5-6 foot tank which is indeed easier.
There is never a guarantee, you could upgrade and still have a relentless Tang. However, a small tank is almost certainly going to have problems now or later with multiple larger fish. Remember these fish come from the ocean and are used to having a lot more territory and grazing area.
Larger set ups cost more $$$, so if money is an issue. I would just return the butterfly or both fish since the tank is pretty small for either.