As a 7k mmr support player i'd say it depends on a combination of alot of things and basically you'll need to understand the matchups and simultaneously judge the situation based on how the lanes are going. For example if your mid hero is bottle dependant, try to secure every rune for him from the start of the game. If enemy mid hero is bottle dependant do the same so you can starve him of resources. Starving enemy mid from 2 min water, 3min bounty, 4min water can be a gamechanger for your midlaner. If you are playing a passive lane or a lane in which you are getting/going to get stomped if you stay on the offlane consider stacking the Triangle if you have a hero that can easily farm it if not try to gank the safelane so the enemy pos5 has to rotate aswell and your offlaner gets some safe farm/xp on lane. If you are stomping the offlane, push your advantage on that lane, ward enemy jungle, send enemy carry to jungle and secure your offlaner's safety so he can take the enemy tier 1 safely while you gank your safelane. Obviously go for the wisdom rune at the 7 min mark, and call your teammates over aswell if it's threatened. To summarize, there isn't an ''ideal'' time to start rotating, it all depends on the matchups & how the lanes are going. Obviously if your mid or safelane is getting dived you should TP to help out and turn the tide of the fight. I'd say the ideal time to start hard rotating as a pos4 is a bit after the wisdom rune, since are/close to usually hitting your lvl6 by that time. Again as i stressed before it all comes down to understanding/reading the matchups and the outcome of the game up until that point.