Gaining

Well, you have a BMI of about 22 now and 30+ is obese. You would need to be 203 lbs to get to a BMI of 30 so you need to gain 52 lbs by Christmas. I think that's going to be difficult. However if you are not too active and can keep up the 4K+ every day until then, you may well go a long way towards your target. Good luck!

It's hard to say if you will get stretch marks as not everyone does. You will just have to find out. I get fine silvery ones on my thighs when I gain but have never had them on the belly.

I would be very impressed if you could maintain north of 4K calories every day without breaks, at your size and likely capacity.

Do you have a hard time keeping it up? Though you did admit to only having been at it for a couple days so far. It's harder to keep that up for a week.

You can expand your capacity but it'll take awhile to increase it, and you'd have to be consistent, which isn't always the easiest thing to do.