Actually, 4 1st rounders for Barzal sound pretty good with 2022 being another strong and deep draft. Go ahead and do that if you are in the western conference with cap space and not yet a top contending team. Isles may be very tempted to do this, it will help them get their prospect depth back to close to where Rags is now in 4 years. They can use one of that four 1st to dump Ladd's contract immediately. Comes worst after the Seattle waiver draft, with all that extra cap space available in 2 years before Belmont opens, they can go after some of the big-name RFA or UFA then. Nelson's line is playing like a 1st line anyway in the playoff and Pageau can handle 2nd line center duty. You can find much cheaper 3rd & 4th line centers in the UFA market or land one via trade or the UFA college route.
The four 1st can actually solve a lot of problems Isles has and since 2020-2021 may be another bump year (gate attendance-wise), losing Barzal may not have that much financial impact and the Seattle draft will weaken the line-up depth of most older contending teams. They can use that space to go after a big name UFA when Belmont opens - new arena and top money can attract players especially those already has an SC ring and has ties with families living in either the North East or European regions.
If the offer sheet is anything less than three unconditional 1st round (2022 must be included) and one 2020 2nd rounder then forget it and Isles will match even they need to overpay a little bit. If they have no cap to do this or the price is not reasonable, they will trade him for either a top prospect center (or a 2022 unconditional 1st rounder) + a 2020 2nd round + a cheap 3rd line center to a western conference team with enough cap space to fit Barzal in their line-up.
Alternatively, Isles can match Barzal to any offer sheet and then make a quick deal with Seattle to take him in the early stage of the expansion draft for a load of high nonconditional 1st round picks. It may be a win-win for everyone at the end. Seattle will definitely jump on board as they need to reach the cap floor and can definitely use an exciting center that they most likely will be able to resign as an UFA (due to his close tie to hometown Vancouver) to jumpstart their franchise.