Splitting DC power supply risk?
Firstly, no you can NOT just throw 4 LEDs in series that happen to add up to the supply voltage and expect them to work - or at least not for long.

If I had £1 for every time I heard someone say that I'd be one rich man indeed!

When above their forward threshold voltage an LED has little or no resistance.  The current that is allowed to flow through it is near infinite.  With no resistor in the circuit too much current will flow and your LEDs will explode.  Literally.  This is a picture of an LED that I removed from a circuit that someone had designed badly by not having any resistors:


So correct your thinking on that front.

Now, assuming you create your LED circuit properly, there is little or no risk to the rest of your setup, as long as the power supply can provide enough current to drive both circuits.

An LED circuit is very passive and doesn't induce any noise or spikes or anything like that back into the power supply circuit.