String assembly based on 4 chars

Projects Programming
2

I would not use Strings at all.

char freq1 = '2';
char freq2 = '7';
char freq3 = 'T';
char freq4 = 'K';

char finalfreq[5];

void setup() {
  Serial.begin(115200);
  finalfreq[0] = freq1;
  finalfreq[1] = freq2;
  finalfreq[2] = freq3;
  finalfreq[3] = freq4;
  Serial.print(F("result is '"));
  Serial.print(finalfreq);
  Serial.println('\'');
}
void loop() {}

result is '27TK'