Every person on tour is going to experience the joys of altitude in his or her way. Some will be surprised that it doesn’t seem to hit them very hard, others may suffer from headaches, nausea and a lack of energy. You have to take the acclimatization process at your own pace and not judge yourself based on other people in the group.
At 2000 meters most people feel a little short of breath, especially when tested by strenuous bursts of physical output, like climbing a big flight of stairs or hiking uphill with a pack on.
At 3000 meters many people feel tired, short of breath, and unmotivated to undertake strenuous activity. This might be coupled with headache and loss of appetite. However, this usually passes quickly.
At 4000 meters you may feel all the symptoms listed above as well as a difficulty to get going in the morning and general irritability. This is normal, welcome to the vertical world. Listen to your body and take it slow, if you feel like you need more time to adjust to the height before attempting your objective, then, by all means, chill out and rest. It will pass if you just give your body a chance to adapt, but if you rush ahead due to time constraints or pressure from other group members you could be inviting needless discomfort.