Yes, if you're using concentrated coolant you should mix it with distilled water. Don't simply replaced coolant with just water though, it's not nearly as effective as coolant/antifreeze, and one of the properties of antifreeze is that it's anti-boil as well, and has rust inhibitors. But as a stop-gap measure, distilled water works until you can get it replaced with a proper mix of coolant/water or premixed coolant.
I wouldn't just ad water. Buy a jug of 50/50 premixed coolant or go to your local Ford dealer and buy some antifreeze.
Buying the correct orange coolant from the dealer is recommended. And adding anything other than that can void you engine 5 years 60k warranty. But if you are out of warranty. U can also add soda it's usually cheaper than water when on sale and will have more anti corrosion additives in it than the water.
At 4 years old you should consider changing the coolant.
Looking for a Used Mustang in your area?
CarGurus has 1,036 nationwide Mustang listings starting at $3,995.
Search Ford Mustang Questions
Ford Mustang Experts
Related Models For Sale
Used Cars For Sale