Bruce left out seven of his first-choice line-up, including leading scorer Abel Hernandez, with one eye on Saturday's trip to Leeds.
They trailed only 1-0 at half-time to Wilfried Bony's goal and with greater ambition could have made life much more difficult for their hosts.
However, Bruce decided not to go for it until he sent on Hernandez after 68 minutes but it all fell apart in the final 10 minutes as Kelechi Iheanacho and Kevin de Bruyne (twice) put the game beyond the visitors before Andrew Robertson's even later consolation.
"The same squad beat Leicester and beat Swansea so I have got to stay true to them if I am going to back my word up and say you are going to play in the cup if you don't play in the league," said Bruce.
"And we lost on Friday (at Derby)
Is it really going to make any difference when you come to Man City?
"If we needed a reminder how cruel it was to play against the big boys, we just had one.
"After 80 minutes we just had our best spell of the match and after 87 minutes it was 4-0
It was never a 4-1.
"The one thing we needed to do in the second half was cause them a few more problems.
"Defensively, apart from the goal, I can't remember them creating a chance in the first half even though they had huge possession.
"We needed to stay in the cup tie
Up until 81 minutes, we were in there
Then they punished us."
Manchester City manager Manuel Pellegrini was satisfied with the result and also that playmaker David Silva made his first start in two months and managed 83 minutes.
"The most important thing was to qualify for the semi-final but after that it was important for David to have 80 minutes after a long injury," he said.
"It was important for Bony to have 65 minutes after a muscle injury and Kelechi a young player to score.
"I am very pleased because I think it was a difficult game against a good team that is at the top of the table in the Championship.
"More than that we played the third game in less than one week and we have a very good response from the team with different names."
Pellegrini may need to use some of those names again for the weekend trip to Stoke as injuries show no sign of easing.
"(Pablo) Zabaleta and (Vincent) Kompany are out, Joe Hart can be fit, I am not sure for Saturday but if not for Tuesday," he added.
"Kun (Sergio Aguero) has a pain in his heel
He is improving and we will see tomorrow or Thursday if he can play on Saturday.
"Fernando has a small problem but I think can play on Saturday
Jesus (Navas) has a problem in his ankle."
Source : PA
Source: PA