What many underestimate is that the squad is not static. Its a living organism were players grow old, they pick injuries, they lose motivation and form or they want out. Now let's have a look at United. We need a RB because AWB is not suitable for a high press, Shaw is injury prone while Malacia had vanished in the treatment room. At CB, Varane and Maguire will probably leave (Varane is old, Maguire is on silly salaries and is unsuited for high press). That's 3 signings in defense alone. In midfield we'll probably need to replace Casemiro and Eriksen and we need someone in RW. That's another 2-3 players. Then we need another striker of course. That means 7 players. We're excluding players who might want out, that Martinez will return to his former best and there's no rotten apples among the top players we would like them to stay (ex Onana, Rashford, Bruno, Martinez etc)
4 signings are good enough when in normal conditions ie when you're squad is competitive and you lack the cherry on the cake. For example the signings of Yorke, Blomqvist and Stam were enough to take the EPL winners to treble winners. Its not suited for a rebuild.
