Read AT commands response string from ESP8266 using Energia and Tiva?

Hi!

I'm trying to receive AT command response string from the Olimex wifi ESP8266 modules (https://www.olimex.com/Products/IoT/MOD-WIFI-ESP8266-DEV/open-source-hardware). The module works well with FTDI and ESPlorer and with linux screen-command. Basically i send an AT command, for example, with "AT" the module should repond with "OK", with "AT+GMR" and the module should give info on the firmware with "OK" at the end. Now, I'm trying to do that with the microcontroller board Tiva C. I wired the pins the following way:

Tiva's PC4 -> ESP8266's RX

Tiva's PC5 -> ESP8266's TX

The GND is the same for all units. I also use an external power supply for the ESP8266 (3.3V).

I uploaded to the Tiva C the following code:

String command = "";
String response = "";
int counter = 0;
int i = 0;
void setup()
{
  Serial.begin(9600);  // USB
  Serial4.begin(9600);  // ESP8266 serial port
  
}

void loop()
{
  
  while(Serial4.available())
  {
     response = Serial4.readString();
     Serial.println(response);
  }
  while(Serial.available())
  {
     command = Serial.readString();
     Serial4.flush();
     Serial4.println(command);
  }
}

I also tried the function Serial4.read() and the results are the same. The LED on the ESP8266-module blinks when I send the command but the response I read are something like:

€þüÿùÿ~ï 

¡ÿüÿøÿï

_Ußôëÿ

I  also replace Serial.readString() by Serial.flush(), added delay(), change the BAUD-rate, send directly the command "AT" with Serial4.println("AT\r\n") (\r\n are needed at the end of the each sent AT-command) But it just doesn't work. Has anyone any idea? Thank you for your time!