Is it safe to use series polarized capacitors to form non-polarized capacitors to be used for higher rated voltage?
I think the question is, how can it be guaranteed that the +/- 400 V is properly distributed between all capacitors.

When two identical polarized capacitors are combined to form one non-polarized capacitor, when charged only one capacitor will take care of the voltage, the one that has the right polarity for that voltage and is charged. We assume that the capacitor with the "wrong" voltage will sort of act like a diode and not charge but also will not be damaged. I have my doubts if this will be the case for all types of polarized capacitors.

Now your case adds that also the voltage must be shared. So 400 V is shared equally between two 200 V capacitors. Both should be charged equally so that no voltage difference will occur.

In theory that could work as the capacitors will behave in an identical way, they have exactly the same capacitance etc.

However in the real world no two capacitors are exactly the same. Also  the behavior of a capacitor will change over time. If one capacitor has slightly less capacitance, it will be charged to more than 200 V. Suppose that slight overvoltage causes more aging and loss of capacitance. Then this difference would get progressively worse over time.

I would not rely on these capacitors to "sort themselves out". I would add balancing resistors and perhaps even diodes (protecting the reverse biased capacitors) so that the effect of mismatched capacitors is minimized.

This is what I mean:







simulate this circuit &ndash; Schematic created using CircuitLab

This would result in a 400 V non-polarized capacitor of 50 mF (\$\frac{1}{2}C\$) where \$C\$ is the nominal capacitance of each of the 4 capacitors.