Advancing con from 4 to 5 raises max hit points from 74 to 82.
The Constitution attribute, or stat, decides the max health of your character. At 1 con the character will have 50 hp, and will gain +8 hp for each added point after that, maxing unmodified health at 274 (level 29 with 29 con).
With added maximum modifiers the hp peaks at 362 with 40 con.
NOTE: There are rumours of guilds that exceeds these boundaries.