Heatsink calculations for a TRIAC
If you are using a heatsink the most important figure is \$R_{th(j-c)}\$ and that value is about 2 degC per watt. This means that if you attach your device to an infinite heatsink it will warm 2 degC per watt i.e. it will become 8 degC warmer for a continual dissipation of 4 watts.

However, you won't find such a perfect heatsink but you might find one that is 6 degC/watt (for example) and you add that figure to the 2 degC/watt figure to give you 8 degC/watt and now, your device will warm up by 32 degC in whatever ambient temperature you have.

However, if the heat isn't removed by some moderate air flow, the ambient temperature around the heatsink will rise maybe 10 or 20 degC. You need to be aware of this and take measurements on the final design but, for now I'll assume ambient will warm by 20 degC.

So if ambient around heatsink is (25 + 20) degC then the device's junction temperature will rise to 77 degC.

There is a slight bonus; \$R_{th(j-a)}\$ is a parallel cooling path from junction to ambient so that figure of 60 degC per watt lowers the 8 degC per watt down to 7.05 degC per watt (resistances in parallel).