How to convert this combination into simple series and parallel?
Look more closely at the original schematic and count the actual nodes.
If it helps, highlight them in different colours.

Note there are actually only three nodes, unique voltage points, in the circuit.
As such you can redraw the circuit starting with the battery.



simulate this circuit &ndash; Schematic created using CircuitLab
As you can see, both R1 and R2 begin and end at the same nodes so are in parallel.
R4 is simply in parallel with the battery.
TIP: Whenever you are given assignments like this be on the lookout for things that let you redraw the circuit. The assignments are intentionally made obscure to make you think.