How secure is the shed? Is the switch in the shed likely to over heat, get
full of spider webs etc? What would the costs be of buying another POE
switch for the shed and the electricity to run it be compared to running
three extra cables from an existing switch?
Unless the cable costs are crippling I'd go with running four cables. If
the single cable is broken, you'll have to run another cable anyway. Having
four cables gives you more available bandwidth if it is needed (now or in
the future).