1.声明变量：

    @property (nonatomic, strong) UITextField *textField;  

2.添加监听：

    [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(textFiledEditChanged:) name:UITextFieldTextDidChangeNotification object:self.textField];  

3.在监听中，过滤非中文字符，并且限制中文字符长度：

    - (BOOL)textFieldShouldReturn:(UITextField *)textField{  
        [textField resignFirstResponder];  
          
        //过滤非汉字字符  
        textField.text = [self filterCharactor:textField.text withRegex:@"[^\u4e00-\u9fa5]"];  
          
        if (textField.text.length >= 4) {  
            textField.text = [textField.text substringToIndex:4];    
        }  
        return NO;  
    }  
      
    - (void)textFiledEditChanged:(id)notification{  
          
        UITextRange *selectedRange = self.textField.markedTextRange;  
        UITextPosition *position = [self.textField positionFromPosition:selectedRange.start offset:0];  
          
        if (!position) { // 没有高亮选择的字  
            //过滤非汉字字符  
            self.textField.text = [self filterCharactor:self.textField.text withRegex:@"[^\u4e00-\u9fa5]"];  
              
            if (self.textField.text.length >= 4) {  
                self.textField.text = [self.textField.text substringToIndex:4];  
            }  
        }else { //有高亮文字  
            //do nothing  
        }  
    }  
      
    //根据正则，过滤特殊字符  
    - (NSString *)filterCharactor:(NSString *)string withRegex:(NSString *)regexStr{  
        NSString *searchText = string;  
        NSError *error = NULL;  
        NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:regexStr options:NSRegularExpressionCaseInsensitive error:&error];  
        NSString *result = [regex stringByReplacingMatchesInString:searchText options:NSMatchingReportCompletion range:NSMakeRange(0, searchText.length) withTemplate:@""];  
        return result;  
    }  

4.移除监听：

- (void)dealloc{  
    [[NSNotificationCenter defaultCenter] removeObserver:self];  
}