Most local car hire companies do deals for 3 or 7 day hires and you may find it not much more expensive to hire the car for seven days than for two one-day hires.
Also bear in mind that having a car will enable you to explore the West Coast beaches north from Sagres if you want to.
Downtown Lagos is small so if you intend to stay in one of the hostels and just enjoy backpacker life a car will not necessarily be a necessity.
If that is the case you may find it cheaper to take the train or bus from Lisbon to Lagos and back.
By the time you pay for the car, fuel and toll Motorways (if you come the most direct route) it may well outweigh the cost of a taxi from Lisbon Airport to Gare Oriente and a train to Lagos for the four of you..
There again taxis are often hard to come by at Lagos Train and Bus Station although they have been improving recently.
