You don't want to leave your team to fight 4v5 if they can help it. Ideally, they wouldn't engage in a 4v5 if they were trying to farm things out. Obviously this is out of their control. The question is whether they can hold off a 4v5 while you push another lane and/or whether you really have anything to contribute.
For example, if your team has a victor and a zyra, the enemy is 5 man siege going on mid and mid turret has a fair amount of health. You don't have to come help and, IN FACT, should push another lane and try to get a turret out of it. Your team will wave clear fast enough that your turrets won't take damage unless they all-in them at turret.
If the other team has dive potential, you're a tank and can peel, you should probably be there. Or if you can realistically sway a dive in your team's favor.
If the team is grouping because they're strong enough to contest objectives and pressure the map, then you should go with them and do whatever you can while not letting yourself get killed. In other words, If you're team is ahead and you're not on their level, let them carry you. Don't get caught out farming, feed the other team, and let them get back in the game. Just steam roll and thank your team for the LP.
Looking at your match history, the games where you are behind are typically games where you're playing jungle. Try not forcing ganks that don't have a fairly high chance of success. Also, full clear, then gank until you have a better idea of when ganks are free. You should be farming well enough early so that you're never in that position where your team is telling you to just farm mid game. Because what they're basically telling you is "stop feeding, you're too weak." You need to be able to realize immediately when you're falling behind and change what you're doing. Sometimes this means afk farming for 5 minutes. Sometimes this means Building an armor item you otherwise wouldn't. What you don't want to do is take risks to try to get yourself back in the game when the rest of your team is doing fine otherwise.
tl;dr- If your team has wave clear, they can defend a siege on their own. Take something else. If you can't affect the outcome of a fight, do something else. Make sure if your team is doing fine aside from you, that you aren't giving the other team a chance to get back in the game by feeding them kills taking risks. Let your team carry you.
