Using buck converters is serial

Apology for the ambiguous Topic.

So I have 12V input from an SMPS to drive a circuit that requires 4V and 5V.

Because of circuit limitations, the 12V output feeds a buck converter in which drives some devices along with another buck converter to obtain 4V.

Is there a problem with this configuration in the long run ? or should I rewire the circuit as to 12V output feeds the Two buck converters directly?

Thanks.

This sounds like a matter of connecting multiple buck / SMPS (Switch mode power supplies) in a cascade arrangement.
Your description is rather confusing.
Can you make a schematic of your current and your proposed configurations and post that. A picture of a hand drawn circuit would be fine.

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

Thank you