The Robins were one up at the break and then added three more in the second half, leaving Brown to ponder. "I saw the best of Preston for 45 minutes and then 45 minutes of the worst and we can't afford that.
"The mistake that found us a goal down after a few minutes - that individual error proved costly.
"The front three I thought caused a lot of problems and, having created chances and having decent possession playing as well as we did, to be 1-0 down at half-time was disappointing.
"We were worthy to have scored three or four goals by half-time. The injury to Conor McLaughlin seems to have knocked the wind out of our sails and we gave a soft performance in the second half.
"The bottom line is we cannot afford to capitulate the way we did in the second half. It's as simple as that.
"They changed their system at half-time and it looks like a tremendous victory from their point of view but it's certainly a poor defeat for us.
"You have to be positive - 18 games to go, there is still time, but we have to eradicate these mistakes."
Bristol City boss Keith Millen was happy with the result following five games without a goal.
He said: "After five games without a goal it's fantastic to get four and a win. I feel the first goal was important and it certainly gave our lads a big lift.
"We had to defend for our lives in the first half and I was happy to go in at 1-0 but I felt we needed to get a second goal.
"We changed things at half-time and without doubt goals change games. You could see the confidence in our lads grow with the second goal and Preston's heads drop.
"After that I was delighted with the way we controlled the game. We looked incisive and cut through them.
"To score four goals is fantastic. Keogh worked his socks off on Tuesday and today showed what a threat he is - he was a real handful.
"We don't talk about the league table, we set ourselves goals and we concern ourselves with performances - in the last five games we've looked solid but not had a goal threat.
"So today we have put that right and have to make sure we build on this."