"Don't die of altitude sickness"

That's the first line of a detailed and informative description of symptoms of traveling to altitude. It goes on to say about Acute Mountain Sickness (the mild form of things):

The most prominent symptom is usually headache, and most people also experience nausea and even vomiting, lethargy, dizziness and poor sleep. Symptoms are very similar to a really bad hangover. Acute mountain sickness can be diagnosed using a self-assessment score sheet. If you have recently ascended to over 2500m, have a headache and your total score is 3 points or more on the score sheet, then you have acute mountain sickness. 

Unless my memory is completely faulty or I adapt better this time I can expect this to be knocking on my door at around the 4500m mark. After which the next few days are likely to be fairly unpleasant before it starts to recede. Not getting the sick is the key here - last time I had a head cold when I left Namche Bazaar and arrived in Pheriche 7 hours later having gained 800m in altitude and feeling utterly wretched. Much to the excitement of the BBC film crew who had been waiting like particularly well spoken vultures for somebody to get sick for several days.