int x,y,l;

char numbers[16]={0x3F, 0x06, 0x5B,           // Chars 0-9 A-F array
0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F,
0x6F, 0x77, 0x7C, 0x39, 0x5E, 0x79, 0x71};
char cn[4] = {1,2,4,8};                                      // = bits 0b0001  0010  0100  1000 of  PORTA

void main()
{
TRISA = 0;
TRISB = 0;
CMCON = 7;                                                    // only 16F628 to disable comparators. To be removed if pic=16F84
PORTA = 0;
PORTB = 0xFF;
// --------------------First loop                             simple slow loop to show all chars
for (x=0;x<4;x++)
{  PORTA = cn[x];
    for (y=0; y<16; y++)     
         {  PORTB = numbers[y];
            delay_ms(150);   } 
 } delay_ms(1000);

// --------------------Second loop                        same loop faster

for (x=0;x<4;x++)
{PORTA = cn[x];
  for (y=0; y<16; y++)     
   {  PORTB = numbers[y];
      delay_ms(15); }
}delay_ms(1000);

//--------------------Third loop                            same loop very fast repeated 255 times

for(l=0;l<255;l++)
{     for (x=0;x<4;x++)    
      { PORTA = cn[x];      
        for (y=0; y<16; y++)      
        {PORTB = numbers[y];         
          delay_us(100); } 
      } 
 } 
}

This is simple program (Mikroc) useful to check the integrity of the connections between the picmicro and the four led displays.

The first array is used to store the characters (0-9 A-F) while a second array is used to set properly the PORTA (to switch on a single digit).

A loop is repeated at different speeds three times where we can see (at higher speed) that all four display look switched on at the same time. The loop, in fact, must be repeated 256 times to be readable.

Schematic to download available in the repository