How to get Bpm with 4 heart beat pulses

Question

I'm trying to get an average from three intervals to get bpm. However, when I use the code below the bpm would slowly decrease when used with the sensor. Can anyone please check for what is wrong with the code and for reference I have to modify the below code from the link for 4 beats to get bpm. Getting BPM from the given code

#include <LiquidCrystal.h>

LiquidCrystal lcd(24, 25, 26, 27, 28, 29);

//global variables

int  ibeats, iInterval1, iInterval2, iInterval3, iCount = 1;
unsigned long iFirstPulse, iSecondPulse, iThirdPulse, iFourthPulse;
bool bIgnoreread = false;
bool bFirstPulse = false, bSecondPulse = false, bThirdPulse = false, bFourthPulse = false;

float BPM, fAverage;

void setup()
{
  //initialize USB serial port, I2C bus and LCD
  Serial.begin(115200);         //set this comm up in-case you want to troubleshoot

  lcd.begin(20, 4); delay(2);
  lcd.clear(); delay(2);

  lcd.setCursor(0, 0); lcd.print(" Patient Prober");


}

void loop()
{

  ibeats = analogRead(15); delay(5);//  Read the Pulse Sensor's value.

  lcd.setCursor(0, 3); lcd.print("BPM:"); lcd.print(BPM); lcd.print(" ");// displaying bpm value on LCD

  if (ibeats > 780 && bIgnoreread == false)     //once the signal is high and bIgnoread is false inital condition, inside if statment will start
  {
    if (bFirstPulse == false && iCount == 1) 
    {
      iFirstPulse = millis();   // time when the pulse is high for the FirstPulse
      bFirstPulse = true;      // to turn off if statement 
    }

    if (bSecondPulse == false && iCount == 2)
    {
      iSecondPulse = millis();
      iInterval1 = iSecondPulse - iFirstPulse;   // first interval
      bSecondPulse = true;
    }

    if (bThirdPulse == false && iCount == 3)
    {
      iThirdPulse = millis();
      iInterval2 = iThirdPulse - iSecondPulse;    //Second Interval
      bThirdPulse = true;
    }

    if (bFourthPulse == false && iCount == 4)
    {
      iFourthPulse = millis();
      iInterval3 = iFourthPulse - iThirdPulse;    // third interval
      bFourthPulse = true;
    }

    bIgnoreread = true; // to turn off when millis second is assign to variable Pulse  When the signal is high
  }

  // reseting all the bollean to intial condition
  if (ibeats < 300)
  {
    bIgnoreread = false;
    bFirstPulse = false;
    bSecondPulse = false;
    bThirdPulse = false;
    bFourthPulse = false;
    iCount++;               // incrment to help to diffrent if ststment above
  }

  //if(ibeats==0)iCount++;

  fAverage = ( iInterval1 + iInterval2 + iInterval3) / 3;         // Average for the interval
  BPM = (60000.0 / fAverage);                                    // Bpm calculation

  if (iCount > 4)iCount = 1;                                    // reseting icount to start the cycle again
}
//********************************************************************************************