The League One outfit were a match for their Premier League hosts during the opening 45 minutes and went into the break level at 0-0.
However, the Blues scored four goals after the break to book their place in round five of the competition.
"I think we did very well and we made their life difficult. We looked fit, worked hard and the tactics worked well," the German told ITV Sport. "After the first goal it was disappointing to concede more, I don't think it was a fair reflection of the game."
Next up for the Bees is a home match against Walsall on Saturday.