“Don't try the paranormal until you know what's normal.” - Granny Weatherwax
The iconography is pretty clear: on levels 1-4 it shows the hand (for "income"), on level 5 it shows the star (for "once, and immediately"). [...]
Not only the iconography is pretty clear, but the rulebook itself explicitly warns you.
Rulebook, page 22, Appendix II: Research Board wrote:
Level 5: Immediately gain nine knowledge. Remember that you no longer receive the income from level 4.