For me we have the following issues, providing De Gea doesn't leave.
RB- Valencia is great but can't play every game, even acknowledged by Mourinho that he is struggling to play twice in one week. This is how it goes for players at the top level when they are getting a bit older. We need a back up option that can come in and still provide a threat down that side.
CB- Too many question marks over those we have here and a bad injury record for Smalling & Jones. Rojo has just had major knee surgery. I'd argue that we possibly need two here if Smalling and/or Jones leave. Neither are good enough and I've accepted that after holding out hope.
LB- Shaw has potential but is now stuggling with injuries, fitness and form. Mitchell looks promising. How Jose will handle this one I don't know. To stick with what we have is a gamble. To discount Shaw & Mitchell is perhaps a gamble too.
CM- We're really weak in these areas. We need somebody with good defensive nous and just general all round quality here if we want to get the best out of Pogba and try to dominate the bigger teams. I like Herrera but I still think we need better. He's not a DM and he needs to play in a 3 I think, this is an issue if we sign Griezmann and want to get the best out of him. Carrick is on the wane and Fellaini is what he is.
LW- Martial has all the talent but has had a poor season. He's also arguably a striker. A tough one to figure out really, bearing in mind the talent that Martial has, but we certainly lack depth in the position.
RW- We have nobody. This position is essential for us to spend money on. Mkhi has been awful imo and Mata, whilst having good technique and being a reliable player, just doesn't offer enough threat or dynamic as a player. If we were a possession based team then he'd be ok, but we aren't. I'm almost expecting one of the two to be moved on this summer.
ST- Without Zlatan we don't have a source of goals in our team. Rashford is still too green and Martial doesn't get much of a chance through here. Griezmann as a 10 could replace Zlatans output, but even then we only achieved 6th this year and struggled for goals too much. We probably need another goalscorer too, somebody a bit different to Rashford in style that can give us ootions.
Backup- I don't think we have enough off the bench in some games and injuries to one or two players can drastically change the dynamic of our team. Having to replace Valencia with Darmian, for example, just ruins our attack down that side. Same with Blind/Shaw.
If we are to only get 4 players then i personally don't think it's enough for us to challenge in the league. There's a bit of a gap opened up between us and other teams which could get even bigger following this summer. Obviously we can't solve every issue in one window but I was fully expecting us to go all out this summer even if it potentially meant bedding in players this season and perhaps not competing in the league, but seeing progress. This idea of 4 players each summer is going to take us a long time to get to the required standard right across the board. I've glossed over a few areas too, because in truth I think we have a pretty terrible squad in general (by our own standards and the standards of the other top teams) and I fully expect many of them to let us down again next season.