Electronic – Surprise resistors found in old voltage divider

I'm a grad student in physics that hasn't looked at circuits since early in my undergrad. I was recently handed a voltage divider that my group used to bias a micro-channel plate detector stack as I'll need to build a new one for a new system.

What I'm confused about is the purpose of some surprise resistors (R1 and R4 on the included schematic) as I don't understand why they would need to be included. Any insight would be much appreciated as the person who built this graduated 8 years ago.

As for the bit with the capacitors in the bottom right along with the output labeled 'Sig', that was to read the bias off of the rear micro-channel plate capacitively to provide timing information on a detector event.

R1=1Mohm

R2=4Mohm

R3=470kohm

R4=150kohm

All capacitors are 4.7uF

Vin is a DC voltage that was biased at -2.5kV.

A microchannel plate detector is a radiation detector with many small channels typically constructed out of a lead glass that function as an electron multiplier. When an incident ion/electron strikes the wall of a channel, it kicks up secondary electrons. Those electrons then hit the wall further down the channel leading to a multiplicative avalanche of electrons that produces a readable voltage pulse on the anode. The resistance across the plate/plates is in the 10^9 ohm range, so as far as I understand it, the resistance of the load between each node (Vfront> Vback>Vanode) is approximately that much.