#include <stdio.h>

// 0AAAA0BB BB0CCCC0 DDDD0EEE E0FFFF0G GGG0HHHH

void convert(unsigned char five[], unsigned char four[]) {
	four[0] = (five[0] << 1) & 0xF0  // 11110000
			| (five[0] << 2) & 0x0C  // 00001100
			| (five[1] >> 6) & 0x03; // 00000011
	four[1] = (five[1] << 3) & 0xF0  // 11110000
			| (five[2] >> 4) & 0x0F; // 00001111
	four[2] = (five[2] << 5) & 0xE0  // 11100000
			| (five[3] >> 3) & 0x10  // 00010000
			| (five[3] >> 2) & 0x0F; // 00001111
	four[3] = (five[3] << 7) & 0x80  // 10000000
			| (five[4] >> 1) & 0x70  // 01110000
			| (five[4])      & 0x0F; // 00001111
}

void show(unsigned char string[], int length) {
    int i;
    for (i = 0; i < length; i ++)
        printf (" %02X", (int)string[i]);
}

void convertAndShow(unsigned char five[], unsigned char four[]) {
    show (five, 5);
    convert (five, four);
    printf ("%5s", "");
    show(four, 4);
    puts ("");
}

int main() {
    unsigned char result[4];
    convertAndShow ((unsigned char []){ 0x08, 0x42, 0x10, 0x84, 0x21 }, result);
    convertAndShow ((unsigned char []){ 0x10, 0x84, 0x21, 0x8C, 0x63 }, result);
    convertAndShow ((unsigned char []){ 0x52, 0xD8, 0xD0, 0x88, 0x64 }, result);
    convertAndShow ((unsigned char []){ 0x21, 0x4E, 0x84, 0x98, 0x62 }, result);
    convertAndShow ((unsigned char []){ 0x7B, 0xDE, 0xF7, 0xBD, 0xEF }, result);
    return 0;
}

 08 42 10 84 21      11 11 11 11
 10 84 21 8C 63      22 22 33 33
 52 D8 D0 88 64      AB CD 12 34
 21 4E 84 98 62      45 78 96 32
 7B DE F7 BD EF      FF FF FF FF