Subject BLR error

I have the following problem:

I got a Interbase DB created with Interbase 4.2 (ODS 8.2), which might
already be corrumpted (I don't know for sure and now I don't have the
original anymore).

After that I opened it with IBConsole and Interbase 6.01 but only for
reading the structure.

Afterwards I wanted to do some changes (dropping a constraint) by
using Interbase 4.2, which returned me the following error message:

invalid request BLR at offset 0
-corrupt system table
-unsupported BLR version (expected 4, encountered 5)

I was then only able to do backup with IB 6.01 and will now try to
restore it with IB 4.2 ...

Can someone tell me the relation between BLR version and Interbase
version? Is it possible, that this error occured because I opened the
database with IB 6.01?

I was doing all on a Windows machine.

Thank's for your help.