Assuming everyone is to be kept alive the answer should be 4.

After every day of travel you are left with 4 days ration.
After 1st day one would return – He would distribute 3 days of ration to remaining others and use 1 to return. So remaining three would be back up to 5.
Similarly after 2nd day one would return distributing two to the others to get them back to 5 and keep 2 for his return.
On 3rd day end another would return giving one to you to get back to 5 and keep 3 for his return.
This five will take you through 5 more days needed to complete the trip.