Again cut and cycle property asked.
Since for a graph of 4 vertices, a spanning tree would have 3 edges,
there is no doubt that edge with weights 1 and 2 would definitely be included in MST because with 2 edges you cannot form a cycle.
Now, we are left to choose one edge with weights remaining as 3,4,5 and 6.
We want to make the weight of MST maximum.
Assign 3 such that it forms a cycle and hence it is rejected.
And now for the imagine graph as a cut and edges 4,5,6 passing the cut.
The lightest edge passing the cut is now 4 and hence included in MST of this graph.
Hence, maximum possible weight is 1+2+4=7.