4. The points \(P\) and \(Q\) are at the same height \(h\) metres above horizontal ground. A small stone is dropped from rest from \(P\). Half a second later a second small stone is thrown vertically downwards from \(Q\) with speed \(7.35 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). Given that the stones hit the ground at the same time, find the value of \(h\).