Goals from Reece Burke, Carlos Mendes Gomes and Admiral Muskwe saw Nathan Jones' side triumph, to earn a tie against Premier League Chelsea.
Writing on Twitter, @mikedoney: Good to see the Luton management getting the team / squad correct again rotating resting respecting the cup at the same time.
@DonnyLebowski: Game over once we scored. Fairly easy for a Luton second string that could of gone through the gears if they had to.
"Future is bright for sure. Thorpe very assured and had an ‘old head on a young body’
"The transformation since we returned in Jan has been very striking."
@ajfkosovo: "Not pretty at times but always in control.
"Good win and on to the 5th round."
While former player Brian Horton tweeted: "A great win for the Hatter’s at Cambridge on a great run of results."
Elliot Thorpe was given his debut for the Hatters, with the midfielder getting some rave reviews.
@LutonTownExile: "Biggest compliment you can take, Elliot, is the fact that all they could do was kick you up in the air.
"Classy on and off the ball. Classy composure.
"Welcome to the shirt."
@ChrisSharples20: "We've been waiting to see you play for us and to see what you’ve got.
"You didn’t disappoint. Well played. Looking forward to seeing you play many many more times for us.
"Can’t wait for your first goal.
@FilthyTops: "Great to see you out there holding your own and a cracking assist."
@BLGHatter: "Well done on your debut , keep working hard."
@chrisdalton1977: "Wonderful debut yesterday - hope for many more opportunities for you."
Another player who was given his first outing as on-loan Aston Villa keeper Jed Steer and discussing his performance, @lutonlocky: "Maybe getting ahead of myself but reckon this fella’s gonna keep goal for us in The Premiership."
@DappaD8: "Well done Steer good debut for us!!!
Rate him as a keeper and IMO will be a great goalie like Sluga was just needs to settle first."
@HungryHatter: "Really solid debut, especially the save right near the end."
@jane738: "He was superb."
@RM_Mili_History: "Decent first start for him, the wind did him no favours but looked solid."
With Town now hosting the eight times winners and European champions at Kenilworth Road early next month, after losing at Stamford Bridge in the fourth round last season, there were plenty of excited Luton fans who will actually be able to attend the game this time.
@callumLtfc: "This time with a crowd & we've improved massively since then.
@LutonBhoy: "Can we get a ref that actually know the rules this time??
"2 balls on the pitch, games stops."
@HappyLuton1982: "My brother in the away end me in the home end!!
"Bring it on. Under the lights the old girl will be bouncing."
@Endi54231604P: "Good luck (from a chelsea fan) if lukaku plays you will win."
@Lonelyfire: "The Lanarkshire Kante v the Original Kante.
"A battle for the ages!"