Swansea host the Cottagers at the Liberty Stadium on Tuesday night in a massive game for both clubs. Fulham are sitting just above the Premier League relegation zone on 19 points, while Swansea only have 21 points.
The bottom half of the table is extremely tight, with only six points separating bottom and 10th.
Coming up for Michael Laudrup's men are a trip to West Ham at lunchtime on Saturday, the south Wales derby at home to Cardiff on February 8 and a trip to Stoke the following Wednesday.
Britton told the Daily Express: "We've had a difficult run of games against teams at the top of the table, but the next few games are against sides around us and those are the games you have to win.
"It's almost like a cup final. They're the games you talk about, we've got to get the three points and stop them getting any to make sure we pull away.
"These next four games will define our season. After them, we will know exactly where we stand."
Both Swansea and Fulham made it into the FA Cup fifth-round draw at the weekend. Swansea will go to Everton next, although Fulham face a replay with Sheffield United to determine if they will host Nottingham Forest or Preston. However, the main focus for each side is still the Premier League.
Swansea's Europa League exploits thus far also seem to have had an adverse effect on their performances in the league, with Laudrup having to frequently rotate his squad to cope with the number of games they are playing. At the end of February, the Swans face a two-legged round-of-32 tie against Rafael Benitez's Napoli.
Britton added: "We went through Christmas saying that was an important period and didn't pick up enough points. Now we have to pull together for another crucial period.
"They are teams who are scrapping for their lives and we've left it so it's a must for us to get results."