Did you feel the cold from the top or the bottom? Is your hammock Double Layer? How tall are you?
You are at the edge of your current insulation's ratings. Some like to keep a 10 degree "cushion" so I use a 20 degree bag until it gets a little below 30 degrees, then I grab "the big dog" (0 degree gear). If you closer to 5' 6" than 6", a ¾ length UQ is almost full length for you. You can always put some Reflexit (mentioned above) or other pad near the foot end.
Because you said put the inflatable inside your sleeping bag to keep the pad from shifting, I'm guessing you have a single layer Hammock. Rather than the inflatable, I'd go for an UQP but I know you don't want to buy any more gear. I'm concerned that your pad is interfering with the UQ and without an UQP, you might have gaps - so many parts in your setup.
Then there's the clothes issue. Some advocate less sleeping clothes. There is less constriction. I wear light cotton pajamas - feels good after a hard day on the trail. But if I'm getting down to single digit temperatures - even the 20's - I'd have a balaclava with me for my head/face. I might even add a light, synthetic, sleeping jacket - but that's only when it is way below 20 degrees.
Is there anyway you can test some systems out before the trip - 4 days is a commitment with gear that might not work. You don't even need it to be that cold at home; you'll be testing combinations and hopefully clearly feel it when you've found the winner.
You didn't mention your tarp. How well were you covered? does the tarp have doors? did you stake it down low at bedtime?
Will there be snow? With snow - it's your friend; it's your buddy - you can use a shovel and/or snow saw and create a 3 sided area with snow walls/berms to keep the wind draft at bay..
The last thing - sort of my rant - is all that insulation and ratings are not making heat - you have to make the heat. If you are not generating heat - the insulation has nothing to ... insulate. So that might suggest attention to what/when you have to eat before you go to bed.
Once I had to spend some winter months sleeping in my station wagon. When born, I was a premie at 7 months and was put in an incubator - glass and metal all around. So when I lay in the station wagon, looking up at the ceiling and windows - it was like MOM! It was also interesting to see the frost on the INSIDE of the car in the morning. I soon learned that if my body didn't make heat at night, there was nothing for the sleeping bag to hold in. You can't be passive (too yin) about this.
You can add heat - with hot water bottles, those hand/pocket warmers (be careful, some leak and make a mess). But better if you figure out what evening meal will add the calories and digestion that will assist your body in making its own heat.
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