If it operates normally until it  shifts to 4th (OD) where
it seems to go into nuetral, then your problem is likely a
failed band. This is a common failure.  The only repair is
usually an overhaul because the trans has to come out and
the front section removed.  You are not far from complete
overhaul at that point and, it only makes sense to finish
the job once you get that far.  You will already have paid
for 98% of the time and labor.  You may want to look at a
used trans but, you will probably not get a lot further with
that.  An overhaul that includes a few updates, a couple of
modifications and a correctly installed shift kit will prove
to be the way to go if you intend to keep the car for any
length of time.  These changes are usually minor expenses if
done at overhaul.  If you are concerned about future band
failures, you can have a wider heavy duty one installed for
only  a couple dollars more but, be prepared to know when it
upshifts.  I don't find the firm shift objectionable but,
many do if they like the soft shifts.  A good trans tech
should be aware of all of the usual mods to improve
reliability and performance.