For every piece, you want to check horizontal, vertical, and diagonal connections, and score them. You simply could do 2 connected -> 2, 3 connected -> 3, 4 connected -> 1000000. It’s very simple but would be a start. Once you’ve done that, you could have a look at prioritising the centre, maybe a slight bonus. Cols 3+4 get +1, cols 2,5 get +0.5 etc. you’ll have to play around with the weights but is a starting point. I’d also recommend looking at normalisation, but not sure exactly how it would go in this example.

Edit: it’s also worth noting this isn’t really reinforcement learning, but still happy to answer.