Edit: this information is inaccurate. This is theoretical battery output, but in practice you get about half. See post #7.
Too much or too little depends on the current draw of your lights. If they are anything other than LEDs they will have a large current draw and your light won't run a very long time. As I understand 18650s are Li-ion? If so, 4 of them in series will give you a 14.4V 4Ah battery. That is plenty for an LED lighting system with under 10W draw. If, however, that lamp is a halogen or other high-power type, let's say a 60W, then your battery will only provide power for an hour at most. The calculation is 14.4V*4Ah = 57.6Wh. This means it can provide 57W for an hour, assuming it's full brightness till it dies and without heat loss. But then if you opt for LEDs, let's say headlight and taillight totalling 10W, the number is closer to 4 or 5 hrs of usage before the brightness begins to drop off.
I am personally setting up a dual 12V pack NiMH system, using 20 1.2V 3500mAh cells arranged in two 10-cell 12V batteries which when paralleled will be an effective 7Ah battery. My lights are LED and there will be a switch to change between the two batteries for use and for charging. In my estimation I will get at least a couple hours of light from each before the power starts to drop off. Only the charging must be done separately, so I will have a switch to select between the two batteries - both for use and for charging. I originally planned to use a 12V SLA, but although these are more forgiving to charging and general abusive use, they are much heavier than NiMH. My two NiMH packs totalling 7Ah are 1/3 the weight of a 12V 7Ah SLA.