I have 4 Vector4s that I need to combine with append to create a certain effect in the Material Editor for a Post Process Volume. When I actually plugged them into the append node, an error pops up that says “can’t append float4 to float4.” If I copy the out-of-date node Append4, it has the same error. If I use the append many, it doesn’t actually append them, it just passes through the first float4.
How are you supposed to combine them?
You can’t append v2 & v3 either. That’d be 5 whereas the limit is 4.
Add them, multiply them, lerp them? If you tried to append v4 + v4 you’d get a v8. That’d be a real headscratcher.
RGBA, 4 is the law.
You need to pass 4 4vecs somehow. Use an MPC, or just 4 lines/lanes to wherever you are going. If it’s a function, add 4 4vec inputs and handle it that way.
Pic or it didn’t happen; what are you trying to do exactly? Move information around a single material, out of a material, into another??
unsure what the PPV function does or what it is returning, but emissive isn’t going to take a 16vec 
you would need to break things apart and do whatever maths you want and then shove an actual color there, 3vec.
4vec is a standard as Alpha was/is a thing and the multiplies, subtracts, etc will will on a pair of 1vecs, 2, 3, or 4vecs as appropriate, but that’s kind of where it fits in. kind of a hardware-software standard, unless you code it otherwise in your own shader or custom-node. Unreal will stick to the rule-of-4.
ultimately, however, what you plug it into rules, so in this case, emissive is a color, like basecolor, so alpha doesn’t apply and we can go with just 3vec.
for efficiency and other things, like bundling values to ensure they correlate over maths, using an append-vec makes sense, but still, at the end, what you plug it into will only take that Xvec value. for example: I pack 4 height-textures into a single texture so when I generate UVs I can reduce samples/samplers (overhead). when i need to height-blend them, however, I need to break them out and do it sequentially, so sometimes it only carries you so far.
