Heat the milk up to 72 °C-80 °C and keep the temperature for 20 min. This will eliminate any competing bacteria which would prevent the yoghurt from thickening.
Take the pot off the heat and let cool to 107 °F/42 °C.
Separate two ladles of milk and pour them into a small bowl. Stir in the 4 tablespoons of yoghurt until smooth.
Add the milk yoghurt mix back to the warm milk.
If you like your yoghurt extra thick, keep it on the stove for another 15 min, at 107 °F/42 °C.This is just my personal preference but this step can be omitted.
Pour the mixture into a flask or yoghurt maker and let sit for 5-8 hours. Don't stir during that time or it might not thicken.
Notes
Always keep some yoghurt from the previous batch to start your next one. Rule of thumb: You will need to add 1 tablespoon of yoghurt for each cup of milk.