Originally Posted by
foxeri
If you are only planning to hit 70 in the season 4 I highly suggest waiting like 2-3 days ater the launch of season 4, after that you should have no problems finding a kind soul to boost you to 70 in t6 game. The progress without exp gem etc takes maximum of 1 hour from even relatively poorly geared booster. There are multible boost communitys you can find like with the search function so finding a booster should not take longer than about 15 minutes.
Now, if you want your rewards instantly on the same day and want to level the character yourself i'd say the fastest leveling would be for first about 20-30 levels farming halls of agony (due it's high density of mobs) Or similar high density areas like fiels of misery (both in act 1) But that can easily get very booring very fast.
Act 1 bounties in overall are the fastest from bounties and finishing bountys in higher difficulities grants high amount of experience.
Also in season 4 entering rifts does not require riftstones so if you don't want to go trough the trouble of recreating game and teleporting from place a to place b, you can just simply keep running rifts on like hard difficulity.
All of the above methods should be relatively effective and get you to 70 in around 4-8 hours, but like said at the start, I highly suggest just waiting few days and letting others do the leveling and get boosted if you are only after the hots benefits, hope this was any help