I need to use a big unsigned char array,but the Arduino IDE compile not passed because the array is too big.How can i solve this problem?
The array has 4M elements.

Is it an immutable type of data, meaning that the bytes won't change along the execution of the application?
In such case it can be stored in the Flash, by declaring it as
const char Array[4 * 1024 * 1024] = { byte1, bytes2, ... , byte4194304 };

It is also necessary to correctly set the Partition Scheme to have an application partition that can store the firmware size plus the 4MB data.

Otherwise, if the data must be allocated to RAM, it shall go to a PSRAM sized to store such volume of data.
Declaration shall be something like:

char *bigArray = NULL;
void setup() {
 bigArray = (char *) malloc(4 * 1024 * 1024);
 if (bigArray == NULL) {
 log_e("Memory allocation in PSRAM has failed.");
 }
 // populating the array with sequence values
 for (uint32_t i = 0; i < 4 * 1024 * 1024; i++) {
 bigArray[i] = (char) i & 0xFF;
 }
}