Since there are so many changes incoming in 5.4 concerning hunters, including removal of readiness, talent changes, a baseline interrupt, ability rebalances,... I feel like if I start to level my 27 hunter this patch, i'll have to relearn him next patch, and the best way to relearn for me personally is during leveling.
Yes, they are changing hunters in 5.4 but it won't be a massive overhaul. The only things likely to change that will affect the way we play will be the opener and our shot priority might change slightly. They're not looking to change our playstlye massively, just a way to make us stronger in PvE and weaker in PvP.
Level your hunter if you enjoy it, because the chances are you'll still enjoy it in 5.4 and you won't need to learn anything new