I think most people are evaluating him as a normal player, not a 4th outfielder.
As a 4th outfielder he is protected by his platoon splits as he has consistently done better against LHP, and he should be facing primarily LHP except when the schedule or an injury requires otherwise, so we are replacing our weak L/L matchups of our three outfielders with him (he has never had platoon protection, and has gone 628, 584, 632, 542, 628 the last four years which is pretty consistently getting more PAs against RHP).
Also, he has been sliding a little on defense, but that has been primarily as a center fielder. The list is long of players who grade slightly negatively as a CF and above average as a corner OF. Fenway expects a little more out of a RF than most parks, but his mark isn't too high to be a net positive defensively playing three positions.
We have to see how the season plays out, but facing lefties for the most part and spreading his innings across three defensive positions somewhat equally and I see this as a really solid potential pickup. Add in the strategy of the extra offensive player on the roster, and there is a chance he can be even further put in positions to succeed, rather than be measured on where his abilities as a 628 PA CF would land him as a 31 year old. So don't compare the 2020 Red Sox Pillar probably usefulness with a 2015-2019 full time CF Pillar when figuring out what he can contribute.