Unfortunately its probably internal. Fourth gear is applied with a clutch pack that can have a number of ways to leak including the nylon seal ring that seals the pack the shaft it sits on. The band that is always engaged till fourth gear also can leak down the application pressure as it is driven off.
A skilled valve body builder could probably open up the flow to fourth gear through accumulator spring pressure changes and/or valve body plate hole diameter. In the long term it likely will have to come back apart.
Bosch Master, ASE Master, L1
33 years MB technician