I hope the error report was suggesting your records are longer then 4000 rather than 400 characters. 4000 is the increased line width provided with V8.
It sounds like you need to use the PREP utility on your source file. Either the file has very long lines or, more likely, the file format has no 'returns' to mark end of lines. What is the source of the file and what formatting should it have?
For V8 PREP should be available from the Program Menu but can also be run as a stand alone program (It's a DOS program so you need just a little DOS knowledge to be comfortable using it.) Details about what it does and how to use it can be found in the Help section provided.
If the file SHOULD HAVE line feeds but they are not recognised for some reason you could consider converting the existing marker to something that Monarch would recognise. This might be possible using and Editor or the Monarch provided MSRP.EXE utility. At this stage I am beginning to try to double guess what your file is so it probably best to leave it for you to identify whay the file is not being read as you expect.
Let us know what you discover!