Good job there! it works on my fiji cards. But you say only 4GB cards, so AMD 3GB cards are out of the game? I thought 144,5 had been carefully chosen precisely to raise the memory usage significantly but still allow 3GB cards to mine.
edit: rephrase
I was not aware of AMD 3G GPUs ... well ... they could work, just not tested it ^^
I just got news that AMD GCN 1 Tahiti cards generally work, just put this file
https://drive.google.com/open?id=1pN-zUv3pvJvrsETFbQ-8j_3gxercIObV into same directory as the other ones

Build Log: Error: AMD HSA Code Object loading failed.
I tried with the 18.6.1 Beta driver.
That bug is stange ... but had that error message before with other Windows build kernels as well ... hmm ... you can try using the Vega kernel included in the Linux package (it is different) ... if that is still not working I will try to call back my testers so I can compile a vega kernel on one of their systems.