Two solutions if the player returns:
1. Have the character play at the 4th level, and take heart in the fact that the new player will advance more rapidly than the others. You have not mentioned that this is Pathfinder Society, so it is assumed that experience points are awarded as in the core rulebook.
A character at the midpoint of Level 7, Medium Advancement, has 43,000 xp. The laggard at the midpoint of Level 4 has 12,000 xp. One-third of Level 7 is about 5,300 xp.
So, after the first mixed-level session, the rest of the players are at 48,300 xp while the Level 4 character gets promoted to Level 5, with 17,300. The part-timer should catch up in a few sessions, especially when reality forces the regulars to take a week off.
Session 2: 5300 xp award, Regulars 53,600/L8, Part-timer 22,600/L5
Session 3: 8000 xp award, Regulars 61,600/L8, Part-timer 30,600/L6
Session 4: 8000 xp award, Regulars 69,600/L8, Part-timer 38,600/L7
Session 5: 8000 xp award, Regulars 77,600/L9, Part-timer 46,600/L7
Session 6: 10,000 xp award, Regulars 87,600/L9, Part-timer 56,600/L8
2. The part-timer gets gold equal to the amount that he would have received during the adventure. Then let the player roll d%. Should the player roll 01-80, the character gets 5,000 xp. Should the player roll 81-00, the player dies but can be raised under the usual conditions and costs. (Other players can chip in, if they like.) The player may repeat this roll until his character catches up to the group, or may stop at any time.