I read 4 to 6 as 6 and less if we are lucky. I read 6 as in ready to skate and start to practice. Torn ligament knee surgeries take a year to get back in the lineup and another year to recover your game.
If you are confident that Kempny is good to go for enough of training camp to be season ready and that the Caps can staff their defense accordingly, you are more confident than I am. Bottom line is that the Caps keep the same 8 defensemen either way
OK, lets say its the full 6, from late March that puts him at late September. Training camp starts in mid Sept, games start the 1st week of October. So if he's back at practice in late Sept, he can still play in a few preseason games. But if he's back in 4 or 5 months, then he has a full training camp. If he has a setback he may miss some games. He will be working out before he starts skating also, so while he may be a little rusty from a layoff, if he gets all his strength back in the leg, he should be good to go.