I have two boys 17m apart and two girls 20m apart.
They definitely went through phases where the sort of focused on working out their play issues and became closer as result. One such time was at age 1-1/2 and 3, and another was at 3-1/2 and 5.
They are very close friends now. My first and third butt heads sometimes (they are 4 years apart) and my youngest is fiesty and also sometimes butts heads with any of the others, but for the most part they are devoted playmates.
I think one of the keys is giving them plenty of time to "just play" with each other, for hours on end. We homeschool and I think that has made a difference in how close my kids are. The more time they spend together, the more they will work out how to get along. Of course it helps to coach them through the tough spots and honor each other's individual needs (I have two kids who, when provoked too much, disengage and have to be by themselves for a bit to recover, and one who wants to hash things out immediately, so they have had to learn to work with those personality conflicts.)
We don't have a ton of toys in our house, at least not compared to most mainstream families we know. If all we ever had was a basket of baby toys (rattles and teether dolls), stacking blocks, soft balls for throwing, wooden blocks, little guys/animals/trees/etc, wooden train, little kitchen, dolls with clothes, a few stuffed animals, dressup, outside balls, ride-ons/bikes/scooters, art supplies, games, and books, I think that would easily have sufficed. Over the years I pared down to just these and they seemed to fill everyone's needs quite well (and still do). Usually if someone gives them some kind of specialty toy as a gift, it gets used once in a blue moon and discarded. The simple stuff is what gets used over and over and over. Also sticks and rocks when they are old enough to play safely with them.
It was really, really, really hard when they were babies/toddlers so close in age, with four in carseats who couldn't do their own buckles yet, and frequent night-wakings, and multiple kids nursing, or at least needing in-arms time, at once. Around when the youngest turned 4 it got significantly easier and has remained that way. I even think I have it easier now than friends whose babies are spaced farther apart (though I didn't think so when they were tiny). Good luck.