What do mean by "Would you use the same number for all citations everywhere, or use a true local phone number if the citation is from a local source?" A citation by definition would be specific to one of the 4 offices. If you're talking about a citation specific to one of the four locations, then you really should be using the local number, for a few different reasons. According to
Google's Guidelines:
"Use a local phone number instead of central, call center helpline number whenever possible."
You don't have to search too far though to find local businesses using 800 numbers and still ranking, or numbers with an out of state area code (from business owners who've crossed state lines with their business, but kept their old phone number). In practice, I haven't seen anyone take a meaningfully large dataset and see if a non-local number actually correlates at all with ranking. I'd personally avoid using an 800 number when possible, and I'd consider leaving an out of state local number if there was already a large citation footprint for that number (I don't know for sure that a non-local area code correlates at all with ranking, but I do know that if you don't do a proper citation cleanup, the inconsistency will definitely hurt you).
The bigger issue, you can't share phone numbers across multiple locations without inviting problems. Do NOT use the same 800 number for citations relating to all 4 locations. Again, in practice I've seen this done too (US bank ATMs for example) but I personally don't have experience doing that, and I wouldn't try before getting a second opinion from someone who's done that personally once or twice and knows what to expect.
My personal recommendation: that 800 number will have it's place, but be careful when and how you use it. Never use it anywhere in combination with an address, and don't use it on the site anywhere that's associated with a specific office. Maybe have a separate part of the site even for national business, vs the 4 locations? Normally I'd recommend making the home page locally related to the main location, but depending on the main keywords the site's already ranking for, you might want to make a different choice there.