That code executes 4 times.

That's not how you do things in Swift:

  class Dwizzle {

  	private let countQueue = DispatchQueue(label: "hey debugger! I'm here!")
  	private var count = 0 //type inference to Int

  	func fizzle() {
  		countQueue.sync {
  			if count > 3 { return }
  			count += 1
  		}
  	}
  }

See, no need for mutexes, or defers, or that lock_guard.

Just simple closures you already know.

Now you want to expose the count variable to other types, always in a non-negotiable thread-safe manner?

  	private let countQueue = DispatchQueue(label: "hey debugger! I'm here!")
  	private var _count = 0
  	public var count : Int {
  		get {
  			return countQueue.sync {
  				return _count
  			}
  		}
  		set(newValue){
  			if newValue >= 3 {
  				self.countQueue.sync {
  					self.count = newValue
  				}
  			}
  		}
  	}

And maintain the rest.

Now what if you want to clean up, and need to only one fizzling executing at a time in the whole process? Doesn't matter which process calls the fizzling?

  class Dwizzle {

  	private static let countQueue = DispatchQueue(label: "hey debugger! I'm here!")
  	private var _count = 0
  	public var count : Int {
  		get {
  			return Dwizzle.countQueue.sync {
  				return _count
  			}
  		}
  		set(newValue){
  			if newValue >= 3 {
  				Dwizzle.countQueue.sync {
  					self.count = newValue
  				}
  			}
  		}
  	}

  	func fizzle() {
  		if count > 3 { return }
  		count += 1
  	}
  }

To me, your code looks like a semaphore, let me introduce you to my over-the-knee fully-functional semaphore that can implement your Dwizzle class:

  class HackerNewsSemaphore {
  
  	private let queue = DispatchQueue(label: "sem queue")
  	private var count = 0

  	func up(){
  		queue.sync {
  			count += 1
  			if executeWhenUp && onlyExecuteWhen(count) {
  				execute(count)
  			}
  		}
  	}

  	func down(){
  		queue.sync {
  			count -= 1
  			if count > 0 && executeWhenDown && onlyExecuteWhen(count) {
  				execute(count)
  			}
  		}
  	}

  	var executeWhenUp : Bool
  	var executeWhenDown : Bool

  	var onlyExecuteWhen : (Int)->Bool

  	var execute : (Int)->Void

  	init(whenTrue: @escaping (Int)->Bool, whenUp: Bool, whenDown: Bool, execute: @escaping (Int)->Void) {
  		self.onlyExecuteWhen = whenTrue
  		self.executeWhenUp = whenUp
  		self.executeWhenDown = whenDown
  		self.execute = execute
  	}
  }


  //test
  let dwizzle = HackerNewsSemaphore(whenTrue: { (i) -> Bool in return i <= 3},
                                    whenUp: true,
                                    whenDown: true,
                                    execute: { i in print("fizzle \(i)") })

  for x in 0...10 {
  	dwizzle.up()
  }

  for x in 0...10 {
  	dwizzle.down()
  }

  //output:
  fizzle 1
  fizzle 2
  fizzle 3
  fizzle 3
  fizzle 2
  fizzle 1