If we only end up with the 4 aforementioned players, i think it would be decent. My thoughts are that these players change the balance of the team. They provide solidity in defence and allow us to have more variety/options in attack. More flexibility. I can imagine a switch between 4-3-3 and 4-2-3-1 and even 3-5-2 easily. It would hopefully help us to make more use of our existing forwards.
I understand many people are clamoring for more world class wingers/forwards/10s like Griezeman, and there is no harm in adding quality to the squad, but we already have so many players in those positions. The more important thing for me, would be to see how we can make better use of those existing players.
From what I can see, the key thing about Perisic, if everything that is being said about him is true, is not that he is world class. He obviously isn't. The key point is that he offers something our existing forwards do not provide. Width on the left and tracking back.
Fabinho and Lindelof strengthens the defensive area of the team's spine. Perisic provides width and tracks back, allowing us to play Shaw and let him push forward. This means that we can have both fullbacks pushing on either side, stretching the play and hopefully this creates more space not just for Morata, but also for Pogba, Mkhitaryan, etc to score goals.
It's also more variety than what we have currently where everyone wants to cut in. Martial, Mata, Mkhitaryan, even Rashford all cut in. Making us predictable and the central area congested. With the new purchases, we could go wide on the left, or go wide on the right or go wide on both sides. Less predictable and more options to create more space and target opponent weaknesses.
Morata, would also provide more pace on the ball, allowing us to play quicker counter attacks.
All in all, I think the aim was to have better balance, more width, more pace and more variety and I think these players achieve that.
Well that's how I imagine Mourinho's intentions, anyway. Just my 2 cents on the topic.
Edit: Also, for Lindelof, it's not just his age and physical profile that fits the team. He is also, by all accounts a ball playing defender, which again, offers something different to the team.