I totally dont mean to shoot down the above but absolutely no way could a SG2 be VAR, for right or wrong, there is such an ego trip amid elite referees' that am sorry, Taylor (has already proved the point at Fulham), Oliver, Atkinson, are simply not going to accept a lowly SG2 officials take on a KMI, that's the harsh reality of it.
SG2 as 4th, yes, indeed, 100% agree, its totally counter productive having Mike Dean as a 4th anywhere, when this role could be better facilitated using up and coming refs who are in line to referee in these stadiums, with these players and these crowds. And who better to learn off than a top elite ref
VAR is best suited to, Mariner for example, coming to end of a respectable career and has been in the exact situations the on field ref will be in, some SG2 who has refereed at Hull City is, with respect to them, going to be instantly fobbed off should he shout "penalty kick" to Micheal Oliver in the league decider.
That (sadly?) is how the chain of command operates.