Lightweight tent recommendations?

I got a hilliberg nallo off ebay

If we wanted a warmer tent, like a 4-season thing that isn't stupid heavy where would you look?

I used Terra Novas in N. Wales and Scotland during the 1980s and 1990s, including winter. Their Voyager might suit your needs. These days I use Hillebergs as they are faster to pitch and lighter, but I do miss the quiet of the taught inner-pitching-first designs of the Terra Novas.

MLD make larger pyramid designs that are rated as 4-season; their Supermid with inner is light and would have reasonable space for 2 people. It could also be used in warmer months with just the outer and a Tyvek floor. There is also the Nigor Wickiup 3.

Forget single-skin designs for winter, it is too damp in the UK for these designs to function without dreadful condensation. Plus they are colder as they lack the layer of air trapped between the inner and outer tents. I have a Hilleberg Rajd and I would only use it in dryer climates/periods of the years. Some folks have been experimenting with Lightwave Sigma single-skin tents and their 'breathable' X-tex fabric. This material uses activated carbon in the coating to reduce the formation of droplets and improve breathability, but under certain conditions it is reported to get very damp and take a long time to dry out.

Are you looking at a 4 season tent because expecting some snow load?
There won't be much difference warm-wise between 3 and 4 season tents.