The only important part of the engine serial number, regarding engine identification, is the three letter prefix. The rest of the serial number is just the build sequence number.
In your case, FEA is a prefix for the 235 used in the 6000 series trucks. The standard engine for the 4000 series trucks was the 216, but the 235 was available as an option (RPO 225). However, I believe the 235 option in the 4000 series should have an A preceding the prefix (AFEA).
Check the link that Baldeagle gave just in case somebody screwed up the serial numbers. You can also look for the block casting number for further verification.
