OK. Have a look at this: Is it okay to use a buck converter followed by another buck converter.
I'd power the "4V" buck via the "12V" SMPS.
But I guess that, as the load changes, there will be a period of instability as the various power supplies "fight" with each other to reach some equilibrium.
For a more rigorous technical analysis, look at this: https://arxiv.org/ftp/arxiv/papers/1802/1802.07159.pdf