question about depth_merge's index

[joye]

@asherliu in https://github.com/iHeartGraph/iBFS/blob/master/inspector.cuh#L118,

depth_merge[wid*vec_sz*num_agg_bfs+i*num_agg_bfs+j*vec_sz+LANE]=LEVEL;

the depth_merge is allocated in Allocator.cuh with
const size_t merge_depth_sz = sizeof(depth_t)*vert_count*num_agg_bfs;

I cannot get the relation with the depth_merge index and whole size, especially variable j in the loop, which is from 0 to 3.
for(int j=0;j<4;j++)

And another question, why each vertex need uint4 data type for status check?