Arsene Wenger left out a few key players in his starting lineup, including the likes of Jack Wilshere, Theo Walcott, Santi Cazorla and Kieran Gibbs. Nonetheless, it was Arsenal who took the lead in the 16th minute through a superb strike from Olivier Giroud, before Brighton equalized in the 33rd minute courtesy of Arsenal's atrocious defending of a corner.
The Gunners took the lead again in the 56th minute, with Giroud netting his fourth goal in two games to make it 2-1, but Brighton proved their quality to equalize just six minutes later through debutant Leonardo Ulloa.
At 2-2, it was Arsenal who looked most likely to score the winner, and when Arsene Wenger threw on the big guns in the shape of Wilshere, Walcott and Gibbs, Brighton's fate seemed all but sealed.
Indeed it was, as Walcott was the man to score Arsenal's winner in the 85th minute.
Here are player ratings, out of 10, for the Gunners as they progressed to the fifth round of the FA Cup.
Wojciech Szczesny: 6
Arsenal's Polish shot stopper made a brilliant one-on-one save in the first half to deny Brighton the opener, but was at the center of Arsenal's terrible defending on the Seagull's first goal.
Carl Jenkinson: 6
He made a few nervy clearances early on, but as Carl Jenkinson settled in the game he began to look more comfortable, even getting a shot on goal in the first half. This was yet another promising performance from the young right-back as he put in a solid defensive display.
Laurent Koscielny: 6
For better or worse, Laurent Koscielny seemed to be the Arsenal player that I saw the least of during the match. Although Arsenal's defending was diabolical on both of Brighton's goals, Koscielny can't be directly blamed for either one.
Still, Arsenal supporters will hope that Koscielny can soon find his form from last season.
Per Mertesacker: 5
Despite being Arsenal's most consistent defender this season, Per Mertesacker had one of his worst games in an Arsenal shirt against Brighton. Wearing the captain's armband in place of the injured Thomas Vermaelen, he failed miserably to organize the defense on Brighton's first goal, and showed some terrible marking on Brighton's second goal.
Overall, it was a bad day at the office for the big German.
Andre Santos: 4
A disgrace to Arsenal Football Club. Enough said.
Abou Diaby: 7
Although he clearly still looks off the pace, lacking some match fitness and aggression, Diaby put in his best performance since returning from injury, playing an Alex Song-esque ball for Olivier Giroud's second goal.
Things can only get better from here for Diaby.
Aaron Ramsey: 6
Aaron Ramsey put in one of his best performances in an Arsenal shirt against West Ham in midweek, but despite playing the same Mikel Arteta role, he didn't have quite as dazzling of a game.
Tomas Rosicky: 7
Considering this was only his second appearance of the season, Tomas Rosicky put in an impressive performance for Arsenal as he was their main outlet going forward in the first half with his driving runs and incisive passes.
Provided he stays fit, Rosicky will be a great option to bring off the bench or for rotating the squad.
Alex Oxlade-Chamberlain: 5.5
"The Ox" has struggled to find a consistent run of form this season, let alone a consistent run in the first team. Although he's become a fan favourite and clearly has a glittering career ahead of him, Chamberlain struggled in a game where he really could have thrived, playing on the right wing up against Brighton's Wayne Bridge.
Overall, fans will be hoping to see better from the 19-year-old in the second half of the season.
Olivier Giroud: 8
Giroud was easily Arsenal's Man of the Match, scoring another pair of fantastic goals to prove that he can lead the line at Arsenal and be the club's main striker. The real question is, can he do it in the big games, and not just against West Ham and Brighton?
Let's see what Giroud does in Arsenal's massive game against Liverpool on Wednesday.
Lukas Podolski: 7
Lukas Podolski had a tough performance to follow after his goal and hat-trick of assists against West Ham, but the German finally played 90 minutes and had a good game, rattling the crossbar with a superb free kick and setting up Giroud's first goal.
Jack Wilshere: 7
Wilshere's introduction gave Arsenal the spark to push on for the winner, going about his usual business of getting stuck in, making tackles and taking players on. Does Wilshere ever have a bad game?
Theo Walcott: 7
Although it took a massive deflection, it was Walcott who will be credited with the goal (his 17th in all competitions, by the way) that sends Arsenal through to the next round of the competition.
In his short time on the pitch, there were more than a few occasions where Walcott could have—and should have—passed the ball instead of shooting it in order to put the game to bed.
Kieran Gibbs: 6.5
Gibbs came on and gave Arsenal some stability on the left-hand side, whereas Andre Santos was failing spectacularly in scoring 35-yard rockets. That's the difference between a top-class left-back who cares, and a joke of a full-back who doesn't.
While Arsenal got the all-important win, it was far from a vintage performance. The biggest takeaway I see from the game is how badly Arsenal's defense needs to improve—if they let Brighton score two, imagine what an in-form Liverpool or Bayern Munich will do?
On a more positive note, Giroud is going from strength to strength at the moment and really seems to be adapting to English football. It would be nice to see him go on a consistent run of goal-scoring form from here rather than this just be another purple patch.
Many are saying that the FA Cup is Arsenal's only shot at winning a trophy this season, so let's hope Arsenal keep the cup run going, with one eye on Wembley.