Hi Mark.. Welcome to the Forum.
The Aluminum Association publishes Welding Aluminum: Theory and Practice This is a pretty authoritative text and respected worldwide.
It's filler cross reference chart calls for 4145 as the filler of choice for 319 sand castings. With several footnotes, noting that 4043 may be used in some applications and that 4047 may also be used in some applications.
Now if 4145 is doing the job it might very well be penny wise and pound foolish to change fillers to a secondary choice. This time your vendor gave you good advice.
When the text notes that "some appliacations may use 4043 or 4047" the decision maker must look at exactly why a substitution would be made and what risks or benefits occur.
While 319 has excellent weldability.. Cracking can still be an issue... When all other preparation and process controls are being employed; Cracking is generally directly attributable to the amount of alloying incorperated into the filler.
By selecting 4043 you sacrifice almost 50% of the alloying Si, which again is the agent that has the greatest influence on hot cracking.
The other alternative, 4047, has a slightly higher Si percentage than the 4145 you are currently using and also likely is just as costly... But might be a good choice if cracking were a problem with the 4145
It breaks down like this
4043.......5.2% Si
4145......10% Si
4047......12% Si
In my opinion, the change your boss suggests is definatly an engineering level decision.