If you are doing summer/autumn bikepacking and camping then a 3-4 season bag is probably one season too many unless you are a cold sleeper. Although 2-3 seasons, i.e. mid-spring to mid-autumn in the north of Scotland is a bit different to the same time frame in Cornwall. Take temperature ratings with a pinch of salt and do a search for any model(s) you take an interest in.
Conditions that make sleeping cooler: bivvy or sleeping under a tarp; wind; increased altitude; higher latitude; thin sleeping mat; tiredness; hunger.
Conditions that make sleeping warmer: in a tent; thick sleeping mat; well nourished; snug sleeping bag.
Down is warmer weight for weight and packs to a smaller volume than synthetic but costs more and is more susceptible to environmental moisture and body vapour.
You can make things warmer by the use of a bivvy bag outer and a silk liner (the silk liner is also useful in protecting a down bag from the oils and other chemicals in your sweat). Having a bag that is reasonably snug also helps in keeping you warm: if you are in a large bag, every time you move around you push the air that you’ve warmed up out of the bag which is replaced by cold air that you then have to warm up again.
I bought one of these http://www.phdesigns.co.uk/minimus-down-sleeping-bag for bikepacking in PHD’s spring sale but have yet to use it as it only arrived a few days ago.