思路：设置一个textField隐藏，然后遍历输入的内容，展示在四个textField中，代码如下：

#import "DemoViewController.h"

@interface DemoViewController ()<UITextFieldDelegate>
{
    UITextField *topTX;
    
    NSMutableArray *dataSource;
}

@end

@implementation DemoViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.view.backgroundColor = [UIColor whiteColor];
    self.title = @"仿支付宝密码输入框";
    
    dataSource = [[NSMutableArray alloc] initWithCapacity:10];
    
    [self setUI];
    
}

- (void)setUI
{
    topTX = [[UITextField alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
    topTX.hidden = YES;
    topTX.keyboardType = UIKeyboardTypeNumberPad;
    [topTX addTarget:self action:@selector(txchange:) forControlEvents:UIControlEventEditingChanged];
    [self.view addSubview:topTX];
    
    //进入界面，topTX成为第一响应
    [topTX becomeFirstResponder];
    
    for (int i = 0; i < 4; i++)
    {
        UITextField *pwdLabel = [[UITextField alloc] initWithFrame:CGRectMake(90+i*40, 100, 30, 30)];
        pwdLabel.layer.borderColor = [UIColor redColor].CGColor;
        pwdLabel.enabled = NO;
        pwdLabel.textAlignment = NSTextAlignmentCenter;//居中
        pwdLabel.secureTextEntry = YES;//设置密码模式
        pwdLabel.layer.borderWidth = 1;
        [self.view addSubview:pwdLabel];
        
        [dataSource addObject:pwdLabel];
    }
}

#pragma mark 文本框内容改变
- (void)txchange:(UITextField *)tx
{
    NSString *password = tx.text;
    
    if (password.length == dataSource.count)
    {
        [tx resignFirstResponder];//隐藏键盘
    }
    
    for (int i = 0; i < dataSource.count; i++)
    {
        UITextField *pwdtx = [dataSource objectAtIndex:i];
        if (i < password.length)
        {
            NSString *pwd = [password substringWithRange:NSMakeRange(i, 1)];
            pwdtx.text = pwd;
        }
        
    }
    
    if (password.length == 4)
    {
        UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"输入的密码是" message:password delegate:nil cancelButtonTitle:@"完成" otherButtonTitles:nil, nil];
        [alert show];
    }
}

效果图如下：

demo.gif

为毛键盘没有显示出来，各位可以自己亲自试一试，思路来自于其他
简友 ，自己只是按照别人的逻辑再重现一遍，膜拜下。。