Background of 4 images added for each other - swift

Question

Please help.
I have 4 pictures.
I wanna show background with animation.
Each picture should fall over the previous animation alpha. As a result, I got a background of 4 images added for each other.
first show 1.png fade alpha to 1.
until one has not appeared until the end, starts the second image, etc.
var waitTime:NSTimeInterval = 0
var durTime:NSTimeInterval = 4.4
for var i = 0; i < 4; i++ {
let frostImage = SKSpriteNode(imageNamed: "a\(i)")
frostImage.position = CGPointMake(0, 0)
frostImage.zPosition = -1
frostImage.alpha = 0
frostImage.anchorPoint = CGPointZero
frostImage.size = CGSizeMake(256, 384)
self.addChild(frostImage)
self.array.addObject(frostImage)
let w = SKAction.waitForDuration(waitTime)
let s = SKAction.fadeAlphaTo(0.6, duration: durTime)
let a = SKAction.repeatAction(SKAction.sequence([w, s]), count: 1)
durTime -= 0.4
waitTime += 0.2
self.arrayAnim.addObject(a)
}
then
for node in self.array {
node.runAction(self.arrayAnim[ind1] as SKAction, completion: nil)
ind1++
}

Answer 1

I wrote for you but in objective-c, I think you can convert it to swift.
SKSpriteNode *bg1 = [SKSpriteNode spriteNodeWithImageNamed:#"bg1"];
bg1.zPosition = 1;
SKSpriteNode *bg2 = [SKSpriteNode spriteNodeWithImageNamed:#"bg2"];
bg2.zPosition = 2;
SKSpriteNode *bg3 = [SKSpriteNode spriteNodeWithImageNamed:#"bg3"];
bg3.zPosition = 3;
SKSpriteNode *bg4 = [SKSpriteNode spriteNodeWithImageNamed:#"bg4"];
bg4.zPosition = 4;
SKAction *action = [SKAction fadeAlphaTo:1.0 duration:1];
SKAction *wait = [SKAction waitForDuration:1];
SKAction *resetAlpha = [SKAction fadeAlphaBy:0.0 duration:0];
[self runAction:[SKAction repeatActionForever:[SKAction sequence:#[[SKAction runBlock:^{
[bg1 runAction:resetAlpha];
[bg2 runAction:resetAlpha];
[bg3 runAction:resetAlpha];
[bg4 runAction:resetAlpha];
}], [SKAction runBlock:^{
[bg1 runAction:action];
}], wait, [SKAction runBlock:^{
[bg2 runAction:action];
}], wait, [SKAction runBlock:^{
[bg3 runAction:action];
}], wait, [SKAction runBlock:^{
[bg4 runAction:action];
}], wait]]]];