Is it permissible for four women to collect donations to enable a man to perform the Hajj pilgrimage without informing him, and merely stating that it is a surprise they will reveal later to the donors?

What the women are doing is a good deed, and the money donated for Hajj must be spent for its intended purpose. It is not permissible to spend it for any other purpose. Concealing what they do, out of fear of ostentation (riya') and sincerity, is a good intention and an act of obedience.