We would normally like to select the edges in the order — 1 then 2 then 3, getting the weight as 6.
But here, we have to maximize the weight. Go with Kruskal — it's human friendly.
We have to select 1 and 2. It is inevitable.
But now, instead of selecting 3, if we put 3 in a loop with 1 and 2, we won't have to take it. So, 3 is skipped.
Now, select 4.
Hence, $4+2+1=7$