Personally I think there are some arguments in favor. But also there are some drawbacks.

Since you already mention the reason why you should do it. I will argue the opposite.

If, like in your example 7 miles is too much. You probably need to recover from a 4 mile run. And it takes more than half a day for your muscles to recover. To cite the BBC: "Protein is vital for the growth and repair of muscle tissue and after hard training this remodelling can continue for over 24 hours."

I don't think it's very wise to go training while in 'recovery mode'. Pure from an effectiveness point of view 4+3 is less effective than 7 miles.

That being said. I do think it's fun to do a second run. I like to do it too. From a psychological point of view. Why not? Evening sun, getting a bit hungry before dinner, having fun.