Hi @Amanda Coleman I have quite a bit of experience with airbnb and have done a tone of research. I think I can provide some value to those questions. So here we go....
First off, what is your zoning? That will determine what you need to go through. If you're in a residential zone, there will be limitations that you need to deal with. The "accessory short term rental" code variances are for residential zones. You can get a lot of answers by going to the city of Portland website https://www.portlandoregon.gov/BDS/65603
The fees, process and timeline are all answered well on the city website. Hotel taxes are a requirement. Airbnb manages the taxes directly (taking from guests and paying to city) and VRBO just started doing that as well.
Regarding your 4br house.....bigger isn't always better if your goal is to have an airbnb. There is a 2br limit for the "type a" permit. You can do more bedrooms as well as commercial gatherings with a "type B' permit, but type b permits are much more costly and difficult to obtain. I don't know anyone with a type b permit. Yes it is legal to rent a house you don't live in, to an extent as there is a 270 day occupancy requirement for a permanent resident; which really means that you can only rent it as a "whole house" for 95 days out of the year (I may have those days flip flopped, it may be 275 and 90). ADUs have slightly different requirements, where you can have full time residency in one structure and then full year astr in the other.
Going back to the zoning question. Nightly rentals are allowed by code in commercial zones with an occupancy change. The occupancy/use change is a longer process than any of the astr permits, but it can be rewarding for the right property. We have a 4plex in a commercial close to the Moda Center and Convention center. Ultimately we went the occupancy change route to create a boutique "hotel". Our building fills a very specific niche, so it has been successful.
I highly recommend doing this above board. Fines have increased within the past year and now range from $1,000 to $5,000 per fine. One owner in town finally gave up fighting the city and sold a building they were breaking the rules with. They ended up paying something like $72,000 in fines. On top of the fines; short term rentals are a controversial topic in Portland.
All in all, I would do it again for the right property, but I won't consider doing it in a residential zone. Theres too much controversy, too much regulation and too much supply in residential zones. Managing a nightly rental is a ton of work that either requires time or high management fees. That typically doesn't pan out in a residential zone (or at least not well enough for the headache in my opinion).
Hopefully this helps. I gave a pretty lengthy presentation on the truths, myths and such for nightly rentals at a recent RareBird Investor Network meeting, so feel free to ask more questions. I'll be happy to help if I can provide value.
Have a great weekend!