The record length says how many payload bytes follow the header.
Length points to the payload
The record length field says 4 payload bytes follow the header.
length=4
The header itself is fixed here
The record header is 5 bytes. The length field does not count those header bytes.
header bytes=5
Total bytes combine both parts
This toy record has 5 header bytes plus 4 payload bytes.
Summary
The length field is the boundary marker between the header and the payload.
payload bytes=4