You need to go thru immigration (can be long lines), wait for luggage (usually the wait at immigration is longer), customs (usually quick), walk over to domestic (easy, 10 min max, just turn right when exiting the terminal), check in at VN (variable), security (never as bad as i have seen in the US without PRE). 4 hours should be easily doable. You might consider a business class ticket on VN or Bamboo, they have a decent lounge if you get thru quicker and allow you to go priority security).