Not good news im afarid
sounds like 4th is fekked, and prob the selctor fork too.
Mine has just done this and lunched 5 cogs and three slectors in the gearbox, 75 plus vat per cog etc etc ££££££££
Aprilias cogs are/were made of cheese as they have uprated them now.
yes its a cassette gearbox so comes out easily but still££££ in labour im afraid
Been using a QS by anychance?