I'm trying to find a method to determine the minimum diameter of a circle that has four circles inside of it that can be of same or different diameter. Here is an image of what I mean:

I visited the subject on Wolfram found here: Circle Packing -- from Wolfram MathWorld but I don't really understand it.

The way i'm thinking of is to take the largest diameters and then just add those, but that is not correct. Is there some sort of formula that will take the diameters of four circles and then give the minimum diameter of a circle to place them inside of? If not, then how should I approach this?