The number 4 actually indicates pattern 4.
In the green sand engine block casting business the item that makes the form or shape in the sand is the pattern. The resulting form or shape is the mold.
For high volume parts like an automotive engine block there are normally several patterns in use that produce the same part number mold. The external features of that block were made by the mold that was made by pattern number 4. For that same block part number there would also be pattern 1, 2, 3 and maybe 5, 6 and up.
