Use the same number of Terrain cards as the 2 person version and lay them down to create the route the exact same way as you would normally in the 2 person version.
Next, give all 4 players their own set of Action Cards (8 each) to use during the game and place the pile face down as normal in front of each players.
Then, you can combine all the travel cards to create a large draw pile in the middle or just use set of the games travel cards and just shuffle them when they run out.
Finally, add all four player tokens to the starting home card.
Players on the same team can play on either of their pieces during their turn. Make sure to sit around the table next to the opposing team so that each team alternates taking turns.
The game progresses as normal and the winning team is the team that gets both of their player token chips back to the final card (this is the card before the starting home card).
Pick your teammate wisely as table-talk is not allowed during the 4 person version.
Also, remember that the team who went second to start the game gets to have the last turn at the end. If this creates tie (all tokens on the final card before the home card), then the official rule is that the game is played backwards in reverse from its current state. The action cards are single use so in the event of a tie, they are not reshuffled and reused. All players return their travel cards and are given 5 new ones to start the round. Since the game ended with the second team having the last move, they are now given the first move at the start of this tie-breaking round.