Though I'm not the world's expert, I've slept in my Ray Mears hammock for a total of about 2 weeks in March kinda temperature, down to about zero / -2. Never had to use anything underneath me, though I have woken up once or twice with a numb bum. No problem; hop on my side and I'm soon toasty again in my luverly down bag.
BTW, if you're after a 4 season sleeping bag, but don't want to pay several hundred pounds, I'd recommend the (Field & Trek own brand) Stormlite 1000, which is rated down to -20. I just picked up a second for £65 in the sale. Packs down well enough to fit with all the rest of my gear for the weekend in a 30L daypack. Nice.
Ken.