Hi Running Like Daddy
My tuppence worth, It depends on what you are running for.
1. Running for fitness, weight control, for the fun of it etc, then the 6 miles over two runs will give the same effects as 6 miles all together which coupled with your long run would give you around 30-36 miles a week.
2. Running for trainig for 5k and 10k and 1/2m Races, given that you will be doing a 10-12 miler at weekends this should be sufficent. You could probably use one or two of your 4 milers as a speed session (fartlek, tempo etc).
3. Marathon training I don't think this would cut the mustard for marathon training as you will need to get use to being on you feet continously for a long period of time and I think the 20 minute break would be counter productive if you are chasing a time. Although if you are just running to finish you could maybe run this way to work twice a week, do two evening runs of slightly longer time/distance (increasing slowly) along with slowly increasing your longer run on the weekend.
I belong to my first catagory so I am not speaking from experience of marathon training so these are just my thoughts. Some of the more knowledgeable peeps will be along and give you the benefit of their experience I'm sure.
Regards & Happy New Year
MK