I second the idea of staying at the Fairmont YVR the night before. I live in Vancouver and even I do this when I have a 6am flight. I roll out of bed, take the less-than-5 minute walk down to the US departures and I get to sleep an extra hour. Bonus - staying at the Fairmont gives you access to the Plaza Premium Lounge. The front desk will give you an invitation when you checkout and there is one just after US preclearance. They open very early and have a fair breakfast spread. And if you stay at the Fairmont, you can take the Skytrain out there the night before. $2.75 per person or $1.75 if over 65 years old - a lot cheaper than a cab if you travel during off peak hours.

I have Nexus so I cut it a lot closer than the others on this board. With no checked bags, I stroll in at 5:30 and still have time for coffee before the loooong trek to the high E gates (for AS in my case). I did check a bag last time so I made it to the AS counter at 4:59am, dropped my bag at the belt just before security, cleared and then went to the Plaza Premium Lounge for breakfast. It has never taken me more than 10 mins at that time of morning to clear security and US pre-clearance. Without Nexus/GE, the lines still didn't look that long. 4am is easily ample time to arrive for a 6am. Without checked bags, I wouldn't even go that early.