A Brian Graham double helped Ross County to a 2-1 victory over last season’s Scottish Cup finalists Motherwell.
Graham broke the deadlock seven minutes into the second half before adding the second in the 60th minute with the excellent Declan McManus providing both assists.
Motherwell substitute Jake Hastie headed home in injury time but County held on for the win.
The draw for the fifth round of the competition takes place on Sunday following the final tie of the round between Hearts and Livingston.
The Staggies made four changes from the side beaten 4-0 by Queen of the South with Marcus Fraser, Sean Kelly, Declan McManus and Brian Graham returning to the line-up. Billy Mckay, Ross Stewart and Tom Grivosti dropped to the bench, while Josh Mullin missed out.
Ross McCormack, who moved from Fulham to Aston Villa in the summer of 2016 in a £12 million deal, started for Motherwell after returning to Fir Park on loan.
County made an enterprising start with a McManus volley well saved by Steelmen stopper Mark Gillespie before former Dundee midfielder Lewis Spence flashed an effort over the bar.
After a quiet start, the hosts gradually came into the game with Danny Johnson firing a left-footed drive wide of the County goal.
But it was the visitors who looked the more likely to net first with Tom Aldred forced to make a timely intervention to prevent Jamie Lindsay tapping home at the back post with Sean Kelly’s overhead kick from the resulting corner just too high.
Ross County had a half-hearted appeal for a penalty waved away five minutes before the break when Lindsay tumbled to the ground following a challenge from Peter Hartley as he attempted to reach a Graham flick-on.
Motherwell made a change at the break with new signing McCormack replaced by Curtis Main.
The Staggies almost made the breakthrough early in the second half when Lindsay found space inside the penalty area but his low drive was superbly stopped Gillespie.
The overworked Motherwell goalkeeper made an even better save to keep out McManus’ low drive that looked destined for the bottom corner.
It seemed only a matter of time before County took the lead and the goal finally arrived in the 52nd minute when a McManus cross was nodded home by Graham.
The same combination linked up again eight minutes later as the Staggies doubled their lead with McManus’ low cross tapped home by Graham to delight the travelling support.
Graham almost killed off the tie in the 78th minute when his close-range effort bounced back off the crossbar before Gillespie kept out McManus’ curling drive.
Motherwell pulled one back in injury time when Hastie nodded home an Elliot Frear cross but they were unable to force an equaliser in the dying embers.
