A Parameterized Vector4 are being treated as Float3. This prevents users from being able to use Vector4s in an If node during runtime.
This is seen as far back as //UE4/Release4.22 cl 8065155 so this is not a regression.
The If node throws an error. "Arithmetic between types float3 and float4 are undefined"
No errors are thrown, material compiles successfully
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-97794 in the post.