エントリータイミングを示すインジケーターではありません。
単にMT4で指定したボリンジャーσに達したときにアラームが鳴って矢印を表示するだけのものです。初期設定ではボリンジャー2.0ですが任意で変更可能。

//+------------------------------------------------------------------+
//|                                          ニャン吉アラーム・ボリンジャー.mq4 |
//|                                        Copyright 2023, ニャン吉ちゃんねる |
//|                                            https://ニャン吉ちゃんねる.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2023, ニャン吉ちゃんねる"
#property link      "https://ニャン吉ちゃんねる.com"
#property version   "1.00"
#property strict
#property indicator_chart_window
#property indicator_buffers 2    
#property indicator_color1 Orange  
#property indicator_color2 Aqua 
#property indicator_width1 3    
#property indicator_width2 3    
input double BBsigma =2.0;//ボリンジャーσ
double ArrowUp[];   
double ArrowDown[]; 
//+------------------------------------------------------------------+
//| Custom indicator initialization function |
//+------------------------------------------------------------------+
int OnInit(){ 
 SetIndexBuffer(0,ArrowDown);  
 SetIndexStyle(0,DRAW_ARROW);  
 SetIndexArrow(0,242);         
 SetIndexEmptyValue(0,0.0);
 SetIndexLabel(0,"arrow_Down");
 SetIndexBuffer(1,ArrowUp);    
 SetIndexStyle(1,DRAW_ARROW);  
 SetIndexArrow(1,241);         
 SetIndexEmptyValue(1,0.0);
 SetIndexLabel(1,"arrow_Up");  
 //---
 return(INIT_SUCCEEDED);
}
//+------------------------------------------------------------------+
//| Custom indicator iteration function |
//+------------------------------------------------------------------+
int OnCalculate(const int rates_total,const int prev_calculated,
                const datetime &time[],const double &open[],
                const double &high[],const double &low[],
                const double &close[],const long &tick_volume[],
                const long &volume[],const int &spread[])
{
 int limit = rates_total - prev_calculated;
 int n = 2; 
 if (limit > 1) {limit = rates_total - n - 1;}
 for(int i=limit; i>=0; i--){  
  if(i+1 >= Bars-1){continue;}

  double BBUpper = iBands(NULL,0,20,BBsigma,0,PRICE_CLOSE,MODE_UPPER,i);
  double BBLower = iBands(NULL,0,20,BBsigma,0,PRICE_CLOSE,MODE_LOWER,i);   

  if(BBUpper<=High[i]){   
   Alert("");
   ArrowUp[i] =0;        
   ArrowDown[i]=High[i]; 
  }
  else if(BBLower>=Low[i]){  
   Alert("");
   ArrowUp[i]=Low[i];    
   ArrowDown[i]=0;       
  }
  else{
   ArrowDown[i]=0;       
   ArrowUp[i] =0;        
  }
 }
 return(rates_total);
}