I'm a regular user of the Salofalk version of this medication (same medication, different manufacturer) so I'll have a go at answering your questions.
1) Mine doesn't say 2g or 4g, how it works is that each squirt of foam delivers 1g. So you just do do the number of squirts that you need. 2 squirts for 2g is the normal dose. I honestly don't know if 4g of your med will come in a different sized cannister or not. I would guess that the cannister would be the same size but the concentration would be more in the 4g, but I'm just basing that on the Salofalk liquid enemas, which are the same size for 2g and 4g but just have different concentrations of the drug.
2) Yep! Put the applicator on, shake the can, insert up your bum and apply the necessary number of squirts. You might have a different applicator from mine but I have two pieces of advice. First is to add some lube to the applicator. It's not already lubed and it can be a sharp little thing. Second thing is to wash and dry some of the applicators rather than just throwing them out. My little cannister comes with 7 applicators but I can get 8-10 doses out of a can so obviously have to re-use some of the applicators.
3) As above, I get 8-10 applications of two squirts (2g) each out of each can. Don't compare it to the Cortifoam, they are different. I would say that the asacol foam uses heaps less per dose because the force from the can sends it up quite far (that's just my feeling compared to the time I was on Cortifoam). It is essential to do the application of foam like the directions say and not use your cortifoam applicator.
I guess it does take up more room compared to cortifoam, but you really should compare it to asacol enemas, which is what it is replacing. I went overseas for 3 weeks a while back and brought enough Salofalk foam to cope with using it nearly every day if I had to (in case I flared. I usually just do 2 a week for maintenance). That only meant two little boxes in my luggage. Taking 18 or so rowasa liquid enemas with me would have been a real headache. I love this foam!
If I were you I'd get the 4g - since you can't get it normally in the US, might as well get as much of the drug as you can while it is available to you.
I don't check healingwell regularly at the moment but everyone should feel free to send me an email if you have questions about the 5asa foam. It's strange, I've been flare free for 20 months now (due to introducing the two maintenances doses of foam into my schedule) and really happy and living life to the full. However, for the (increasingly rare) times that I do think about having UC, I get more down than I used to. So I've been taking a little holiday from thinking about it! I used to research and read about it all the time when I was sicker and found it quite interesting but now it makes me more down than it used to. It's strange how our mind copes with these things.
Anyway, I'll make a point of checking here in case you have any other questions.
Left Sided UC
Maintenance Meds: Salofalk tablets (Asacol) 2 tablets 4 times daily (4gm total) plus 2 Salofalk foam enemas per week.
Flare Meds: Salofalk enemas and/or Colifoam
Oat bran 60grams daily, Metamuscil and multivitamins