Add constant DC voltage to a variable DC input
This sounds like a good application for a charge pump.  All you need is a roughly 4 V square wave somewhere.  It can be between ground and +4 V, for example.

The square wave goes directly into a capacitor, so there can be a arbitrary DC offset across it.  As long as this offset changes little during individual pump cycles, it can be considered as "constant" for this purpose.

Here is the basic circuit:



Vout will be the amplitude of the square wave, minus the two Schottky diode drops.  For example, it would be about 4.3 V open circuit with a 5 V square wave as input.

C1 allows any arbitrary offset voltage between the square wave and Vout.  Of course C1 needs to be rated for the voltage.  C2 only needs to handle Vout.

Either side of Vout can be used as the reference.  If Vout- is tied to your 30 V rail, than Vout+ will be a little higher.  If Vout+ is tied to your 30 V rail, then Vout- will be a little lower.

The current capability of this charge pump is proportional to the pump frequency.  A convenient place to get a square wave from can be a clock output of a microcontroller that is there already anyway.  You can also make a square wave from a Schmitt trigger inverter with a R-C low pass filter feeding its output back to its input.