As a person who has solo MMR around 4000 - 4400 and regularly playing with my 2000 - 3500 friends, I can confirm that this happens a lot of the time. But that is not because we are 'selfish' or 'no teamwork' but it is more to different in playstyle. You've got to know that different skill bracket has difference playstyle. For example, in lower bracket, if mid didnt gank by level 6, people will go "f*ck mid no gank pls reportz". In higher skill bracket, mid who constantly gank will always lose their game (most of the time). This is because higher skill player knows how to retreat when they see opponent mid player is missing. Hence, the ganking mid will not get any kills and will be underleveled and underfarmed while the nonganking mid will free farming the midlane and in worst case scenario, he will take down the mid tower. Now, I'm not saying that mid that goes ganking is completely wrong. In fact, getting kills awards more gold than farming or pushing, but it's about risk vs rewards. I agree that in lower level it is better for mid to gank because people at this skill level (mostly) still have bad gamesense and decision making. So, killing lane supports or safelane carry is not that hard (snowballing). But in higher MMR bracket? most of the time its not worth leaving your lane until at least you get your core item (for example orchid on Storm)

So why sometime your 4k friend didnt gank? Its because playstyle and habit. Usually they do what they did at their normal (4k) bracket although they are playing at lower bracket (2k,3k). Another example is diving without teamates. This happens to me a lot of the time. When I play bloodseeker in my solo mmr (4k bracket) and have dazzle on my team, I would not hesitate to dive under enemy tower eventhough im against more than 1 hero. This is because I know the dazzle will protect me from dying. We both know that bloodseeker is OP and has the capability to dive and tank the tower while at the same time, rape whatever beneath the tower espcially with the hlp of dazzle. On the other hand, when playing on party mmr (2k bracket), with same situation, I'm expecting the dazzle will protect me when I'm diving. But instead, he panic and run away when he saw im low hp vs 2 half HP hero. Conclusion? sometime we 4k player expect 2k,3k to think the same like us and when it's not, shit happen.

When you play with someone of higher skill level than you, you have to respect their decision. You have to give them the benefit of the doubt. Yeah you might think that they actually made many noob mistakes but they are at their MMR for a reason. They made better decision on most of the time. This is the difference between lower level and higher skill player. It's not about item build, or skill build or KDA. It's the decision making that allow them to achieve thier MMR (with some luck too of course). Higher level skill player usually have different thought process than lower skill player. Now of course, higher skill player do make mistake. Everyone does. But doesnt mean you should put your trust away from them. Usually their mistakes is outweight by their superior decision making.

Oh im not saying 4k is the superior mmr or something. This thing doesnt just apply to 2k,3k VS 4k. This should apply to 4k vs 5k too. or 5k vs 6k and so on. We should constantly learn from each other and espcially from someone better cos that's how you get better  .