Driving 4.2V TFT by 5V
Be careful using a diode for its voltage drop. Voltage drop is a function of current for a real diode. Lower current -> lower voltage drop. Voltage drop is also affected by temperature. I'd definitely use the XL1509.

If your converter is getting hot, measure the input current and output current. Then look at how much power you're losing (Vin * Iin - Vout * Iout). The check the XL1509 datasheet to see if that looks right.