The only truly immortal ones are the third and fourth wife. I don't know the first two's cultivation but it isn't bad (at least not compared to Lin Ming. Everyone is a failure compared to him)
3rd and 4th wife are not normal people ... His first two wives came from Lower world, still they are above almost 99.99% cultivators .. Spoiler Were they nt true god by the end of novel?
Maybe after he became the human emperor and basically ran the entire universe. At least before, only the last two were true gods. The first two, even with perfect godbeast bloodlines, will have a hard time getting past Holy Lord.