I've not used 4643 before, but I understand it leaves a heat-treatable Silicon-Magnsium alloy deposit. So it would be perfect for welding say, 6061 especially when it is going to be post-weld heat treated.

There are actually probably a lot of cast alloys that are heat treated Silicon-Magnesium alloys, so I could see it potentially being useful on those. Often a heat treatable alloy even if not heat treated after welding or casting will "age harden" just with the normal passage of time, to some degree. So yeah, I could see 4643 being useful potentially even for parts that aren't post weld heat treated.

I like 5356 for when good "as welded" strength is needed, although it is not recommended for anything but low sustained temperature applications. In that case 4043 would be typically recommended.

There are other exotic alloys available. I've got some 4145, for example. It's got some copper in it (among other things) and is good for welding some uncommon alloys, especially those having some copper in the mix.

Another alloy for welding some cast aluminum that would fall into the exotic category is A356. That is a pretty close match for commonly used sandcasting alloys.

I know Jody and others have recommended high-silicon alloy 4047 for cast aluminum, I think because it has a lower melting temperature, it can help it "wet out" better. I haven't tried that yet, but would if I could pick some up without it costing too much.

I usually get by pretty well with just 4043 and 5356. Hope this gives you some good food for thought!
http://www.alcotec.com/us/en/support...tion_Chart.pdf