Postgres - MultiXactId error
I would advise to backup first ...  but that's a catch 22, since you can't.

Either way, since the error revolves around a fictional xid wraparound I thought this might solve your immediate problem:

VACUUM FREEZE tbl;


The chapter Preventing Transaction ID Wraparound Failures in the manual has details.

But turns out, this runs into the same problem. 

Of course, you can delete and recreate the table if you have the data outside Postgres. 

The only remaining option I see is to upgrade to the latest point release and hope your problem is covered in the various patches that have been added, like @a_horse commented.

However, after some more research I found that the core hackers have been working on a solution for just this problem in the past weeks.
Details in this thread on pgsql-hackers.
The commit message.

So I would hold back and upgrade straight to Postgres 9.4.9, which should be out soon:
The next point release is planned for August 11th, 2016 at the latest.