Resistors used to compensate for loss of power with LED replacement. OEM configuration is 27W+5W+27W = 59W for US cars and 21W+5W+27W for the EU.Typical power for LED bulb 3-5W. Side markers could be 1-2W, if you replace all 3 - it will be 3+1+3 = 7W in the worst case scenario. You will need to compensate for 50W of power loss, to prevent hyperflashing.
Resistor 6 Ohm will compensate about 30W (at 13.5 voltage)
Resistor 4 Ohm will compensate about 45W
Resistor 33 Ohm - only 5.5 W, which makes no sense, unless you need to compensate for very little difference.
Selecting 4 or 6 depends what kind of LED you are going to use. Specs should have power consumption and you can calculate total power.
I have a blog, explaining how it works in Mazda. fixing-hyper-flashing-with-leds