Serial.print() can print String, char[], int, float, ...
TFTscreen.text() can only print char[]
The clue is in the method name. Mind you, SensorVoltage is a char[]
and you have used dtostrf() correctly:
char * dtostrf( double __val, signed char __width, unsigned char __prec, char * __s)
However you are too mean with your SensorVoltage[4]. You are not allowing for the terminating NUL character.
David.