Are you running NAV 3.xx.y objects on NAV 4.xx.xx executables? In my experience this is a very common configuration, and have not seen any problems, in fact I see it as much more stable that running the 3.70 executables.
There is some information going around that this is a bad combination, and since it seems very popular, I am concerned to resolve the issue as quick as possible. If you are running this (or similar eg 3.60 on SP1 etc) can you post your combination, the country version, db (Native or SQL) and weather it is running fine or not.
For example I am working with a client now that recently upgraded from:
NAV3.60W1 on 3.70B Native to
NAV3.60W1 on 4.00SP3 SQL system is running great.
another client is about to upgrade:
NAV4.00US (no SP) on 4.00SP2 Native to
NAV4.00 (no SP) on 5.00 SQL
My main concern, is that this maybe because of the way 4.00 allows SQL to determine the type of row lock, in which case it may be a SQL only issue.