C++ resizing a 4D stl vector -

i have 4d vector of following type:

vector< vector< vector< vector< unsigned > > > > vec don't know size of 4th dimension @ beginning, trying resize first 3 dimension in following way:

vec.resize(x.size(), vector< vector< vector< unsigned > > > (y.size(), vector< vector< unsigned > >(2))); 

where x , y of type vector<unsigned>

after resized first 3 dimensions in above way mentioned can't access element dimension 3(i.e vec[i][j][1]).

can please suggest missing here?