The first choice back four is really stable, not remarkable but gets the job done.
Shaw -- Maguire -- Lindelof -- AWB
The second choices though.. needs upgrade or better challengers to the first choices.
Williams -- Jones/Tuanzebe/Bailly -- Dalot
Overall,
Starters
Tier 1: Maguire (I mean, there isn't much choice really, not many top CBs nowadays)
Tier 2: AWB (much better attacking quality is the only limiting factor here)
Tier 3: Shaw (he's good but we know what the problems are)
Squad players
Tier 1: Lindelof (he's a damn good 2nd-choice or squad player, can be upgraded easily)
Tier 2:
Tier 3: Williams (top mentality and character, but limited abilities)
Difficult to put where Bailly, Jones and Tuanzebe considering their fitness tendencies. If they're fit, would put all on Tier 2. Dalot also performed like.. idk, no harsh comments, possibly Tier 3 because of purely his more natural attacking talents.
TLDR/
Only the first choice back 4 is "fixed".