I just weighed 4 slightly heaped tsp and it came to 20g. According to the Nutrition on the packet this is 80 calories so 4 syns.
OK I've edited this post as my maths in discombobulating this morning:
Basically 4 heaped tsp is 4 syns, irrespective of how much water you put it in. For some reason SW think it would be interesting to calculate a 142ml serving (which would only need 2 heaped tsp, so is 2 syns). The Bisto pack says put 4 heaped tsp in half a pint (280ml) which is 4 syns.
At the end of the day 4 heaped tsp is 4 syns however much water you actually put it into. About 1 syn per heaped tsp.