I am a member of the Bigfoot Owners Club Intl. and on the discussion forum there are several owners of the 4000 series. Most speak well of them, but they are not without their issues.
If the one you are planning on getting has been well cared for and kept under cover or shelter, it will be a real nice rig.
I would think you could sell your 2017 model and if a clean rig, make almost what you paid for it. Just be a wary seller and double check everything. When I sold my MH in 2019 I went behind the buyer and checked with the credit union they were borrowing part of the money with, to confirm it was a legit deal. I contacted them thru the email address I got off their web site and not via the one the CU employee handling it was using. I gave all pertinent info and simply asked for a yea or nay that the loan was actually being handled and was legit. They said yes. The buyer said he was going to give me a Wells Fargo cashiers check for a part of the price, so when they arrived (flew in on airline) i picked them up in the motorhome and we went to Wells Fargo. Oddly WF could not easily confirm the checks, they needed to talk to the issuing location, which would not open for 3 hrs. However the manager looked at the buyers accounts and saw that that exact amount had been moved out of the account. I also had done a bunch of research on the buyer, where they lived, I checked the property tax records for the address to confirm the ownership, and several other things.
There are some pretty good scams going right now. The very best thing you could do would be to walk into the sellers bank or credit union and be handed a check by the teller for the proper amount.
Charles