City sprung one of the major surprises of the previous round as they defeated Premier League opponents Crystal Palace 2-1 at Ashton Gate to set up another tie with top-flight opposition.
That win was in stark contrast to their league form this season, with Sean O'Driscoll's side seemingly unable to shake off the disappointment of last year's relegation from the Championship.
City, for whom semi-finals appearances in 1971 and 1989 remain their strongest League Cup showing, have struggled to arrest the slide so far this term and they currently occupy one of League One's relegation spots, with just four points from their opening eight games.
The cup competitions have proved a welcome distraction for O'Driscoll's side, who have beaten Gillingham and Palace in the League Cup, while they also beat local rivals Bristol Rivers 2-1 in the Football League Trophy.
The club's league form remains a concern though, and O'Driscoll has pleaded for patience from the fans during what is a period of transition.
"This is going to be a difficult season - I've said it time and time again," he said.
"We're not anywhere near where we should be because we are still starting on this journey."
Midfielder Stephen Pearson will not make the trip to Southampton, after it was announced he will miss the next six months with an ankle injury, but O'Driscoll revealed that Jordan Wynter and Neil Kilkenny are both back in training after recovering from knocks.
Southampton, defeated League Cup finalists in 1979, reached the third round with a 5-1 thumping of Barnsley last month.
They will enter the match fresh off a superb 1-0 Premier League win at Liverpool thanks to Dejan Lovren's second-half header.
Southampton have found goals tougher to score in the top-flight though, netting three in five encounters.
The two sides last met in the competition in October 2003, when goals from James Beattie, Brett Ormerod and Greame Le Saux helped them to a 3-0 win in the third round.
And a similar result looks likely given the respective form of the two sides heading into the encounter.
