Brian Graham netted in either half and Greig Spence and Dougie Hill got the others as the Division One outfit ran County's makeshift side ragged.
A late Russell Duncan goal was all the home side could muster on a night to forget for the SPL new boys.
County gambled with nine changes to their line-up from Saturday's 1-0 win at Dundee, while Raith made only one change from their unbeaten start to the season.
County created the first real opening in the seventh minute when Mark Fitzpatrick got to the byline and his ball into the near post was headed over by Sam Morrow.
In a similar move in the 12th minute Morrow's looping header from 12 yards was easily saved by Raith keeper David McGurn.
The visitors started to get to grips with the tie and Spence's overhead kick in the 15th minute flew wide of target after Jason Thomson set him up, then a low drive from Graham drifted wide of the other post.
At the other end, Kurtis Byrne let fly from the edge of the 18-yard box but his shot went well wide of the far post.
But it was Raith who took the lead in the 25th minute when Graham slipped the ball through to Alan Walker, who in turn teed it up for Spence who had the easiest of tasks of guiding the ball into the net.
The home side responded with a stinging shot from Byrne which was held by McGurn.
County's makeshift team were playing all the football but Raith looked dangerous every time they broke.
Raith were dealt a blow when experienced defender Laurie Ellis hobbled off to be replaced by Colin Wilson on the half-hour mark.
The SPL side were restricted to long-range efforts from set-pieces without really threatening the Raith goal.
The County goal came under threat again in the 37th minute when Pat Clark flashed a shot narrowly wide of the far post.
The Kirkcaldy side forced their second corner of the match in the 40th minute and from Walker's inswinger Graham sent a bullet header into the net at the near post.
Walker got a shot on target seconds into the second half after he was teed up by Pat Clarke but his effort was straight at Brown.
But it got worse for County in the 51st minute when Walker swung over a free-kick from the right and Hill rose above everyone to head powerfully into the net and make it 3-0.
County looked in disarray and it was no surprise when Raith extended their lead in the 63rd minute. Walker's shot inside the box was blocked and Graham latched on to the loose ball and drilled it into the far corner of the net.
Brown was forced to pull off a full-length save from Clarke as Raith piled on the agony.
County got one back in the 84th minute when he prodded the ball over the line after Martin Scott set him up with a cheeky back-heel.