I have been keeping 10 adult 'efasciatus' and 'notatus' in a 90G tank for over two years now (just the way it's worked out). There are two breeding pairs of each species in there plus a couple of extra males. Yes, they're a little crowded, but they spawn regularly and tolerate one another incredibly well. There are a few split fins to be sure but on the whole, it's amazing just how well they're doing. I think you are right on when you mentioned them growing up together from an early age as being a key factor. But I've found that compatibility is improved with both males and females present. I have set up the tank with several sight line breaks (tall plastic plants and drift wood) and have a number of ceramic pots in there as well. I'm not proposing that it's best to overcrowd, but it does spread the aggression around. I wouldn't hesitate keeping the four males together. Here's a video of two pair ('efasciatus' and 'notatus') spawning on two different sides of the same piece of flower pot. It was taken a year ago, and they weren't full adult size (they now are).
[video=youtube;ow_Njq4JYmY]http://www.youtube.com/watch?v=ow_Njq4JYmY&feature=g-upl[/video]
Jim