@Millanrose13 this is similar to how I travel (MacAir, External hard drive). The Air is nice and small to travel with, but as you know the flash drive space is limited so you need an external HD or you'll fill things up quick.
First, make sure your mSD card reader is fast, and buy a spare. Some card readers (even new ones I've bought) will come up and tell you a big transfer will take an hour, then you take it out and use another one and it does it in 5 minutes (and tells you that in the estimate). So, test it home.
The Air can handle 4K files fine, but if you are doing movie editing on your trip, it may not play back smoothly due to the speed limitations of your hard drive connection. You can still edit (it'll be choppy) and when you export it'll be perfect. You can get around this by keeping everything on your hard drive, but then space becomes an issue. When FCPX works (if that is what you are using) it eats up a lot of space with the transition files it creates....it grows quite a bit. (Having said all that, when I get home I transfer everything to my MacBookPro and a faster hard drive for easier editing.)
I have been very happy with GDrives. They work great with Macs and are reliable. I use the ones that are bus powered so they work without needing an external power supply. GDrive makes some pretty big flash storage drives, but they are pricey. I have been using the HD based ones which are a lot cheaper. They make this G-Drive slim which pairs really nice with the Air. You may need a couple depending on the length of your trip and amount of footage you plan on collecting.
Safe travels. Look forward to seeing the footage!
I use FCPX to do the importing. When creating a Library I assign the storage location to the external drive. You do this in the Library Properties window when you create the Library. Any files imported to the Library are automatically stored on the external drive. FCPX on the host Mac uses links to edit the files on the external drive and doesn't use any space on your hard drive. The process Is lightning fast if your external drive has a Thunderbolt connector.
FCPX does all this in the background so you can work on other Libraries at the same time. You also get a lot of metadata such as color grading while the import is being done that will speed up your editing workflow. You don't get any of this metadata if you import through Finder.
I believe you will get similar results and functionality using any good video software tools as well. Well worth the investment.