From what I've read/heard, the sound from the two is very similar. Some think the mk4 is a tad bit warmer and nicer, but it's by a very small margin. The largest difference is the pattern, but I don't know enough to talk about the differences there.
The mk4 can perform very well for interior dialogue, and it's been used quite a few times by many professionals. Since it's wider though it depends upon the surroundings being quiet enough that you don't need the extra directionality of the mk41. Regarding which would sound best for a certain situation, it really depends. For very small rooms, quiet sets, or when a wider pattern would make it easier to boom, the mk4 is very good. The mk41 is probably a better problem solver being narrower, and can do a good job in more situations than the mk4.
