Lightweight 4 season sleeping bag for bike packing - recommendations please

[FrothNinja]

It is the correct bag @HobbesOnTour , I seem to turn into an ice cube as soon as I stop moving about.
Was warm enough at 8 C with my rather heavy fleece liner, fleece jacket & warm socks.
The liner & compression sack squish down to a package 27 x 21 x 16cm & weighs 891g. Can't find my home made ultrafleece liner I used last year but that sleeps warmer & packs smaller.
My mat is a primacore inflatable.

 

[HobbesOnTour]

I seem to turn into an ice cube as soon as I stop moving about.

That'll make camping interesting!
Seriously, though, I'm wondering if there is a difference between being "at home" and on the road? Going from a (presumably) cosy indoor environment to a fresh, cool night outdoors can be a shock to the system. It's not so much that you are uncomfortably cold but that your body temp is dropping and that can feel really cold.
The reality of a day on the bike is that you'll be more acclimatised by being out all day.

Was warm enough at 8 C with my rather heavy fleece liner, fleece jacket & warm socks.

Not meaning to be critical but it strikes me that that setup is overkill for those temperatures.
However, being comfortable is essential for me and a good night's sleep is important to enjoy the next day. You're figuring out what works for you and in my book, that's more important than weight.

The liner & compression sack squish down to a package 27 x 21 x 16cm & weighs 891g

That has to be liner & bag?
Personally, I don't focus too much on weight. Something has a job to do. It's no good to me if it doesn't do the job, no matter how light it is. Sometimes I don't use the compression sack for the bag at all as I think it gives better packing options loose in the pannier.

My mat is a primacore inflatable.

That's Robbens - a decent brand, rated to 2.6 C. But that's not a standard measurement. If you run to cold then anything that can boost that will help.

It seems like your next step is an overnighter in the tent!

 

[FrothNinja]

Still haven't found my ultrafleece liner yet, but while searching for it I found it's predecessor made from Meraklon.
This is a good thing. However....
I made the ultrafleece one because the Meraklon one is just under 60cm wide and top loader only. I did had a prefunctory hood to it back in the olden days.
On the plus side it weighs 450g with its sack but without a compression sack.