Leam Richardson just has not been able to turn the tide since taking over from Matt Taylor in December and rock-bottom Rotherham are now 19 points from safety with only seven games left.
After a rather pedestrian opening quarter of an hour at Deepdale, the hosts created the first opportunity of the game.
Andrew Hughes pounced on a loose ball in the Millers box, only for his effort to be blocked by keeper Viktor Johansson.
Testing weather conditions did not help the flow of the game, but it gradually began to come to life.
Rotherham's Oli Rathbone fired a shot narrowly off target, before Preston struck gold at the other end as they took the lead.
Duane Holmes celebrated his return to the starting line-up, darting in to coolly convert a smart cross from Robbie Brady.
Minutes later roles were almost reversed as Holmes provided a cross for Brady, but he headed just wide.
The hosts had well and truly awoken from their early slumber, and Will Keane was next to go close soon after.
It took the seemingly doomed Millers a while to respond to going behind, and they went close to a leveller after half an hour.
However, Preston keeper Freddie Woodman was agile enough to dive and keep out a thumping strike from Cafu.
With half-time approaching, Preston deservedly doubled their advantage.
This time Riis provided the finish, taking a touch from Mads Frokjaer's cross before slotting smartly past Johansson.
Five minutes later Riis struck again to make it 3-0 and effectively make it game over, as the Dane poked home from close range after a ball into the Millers box was not cleared.
The first chance of the second period went Rotherham's way but Charlie Wyke's header was comfortably saved by Woodman.
Preston, who remained firmly in control, responded with hat-trick hunting Riis firing in a shot which was diverted behind for a corner.
Riis then turned provider as his cross was met by Keane, who glanced a header inches off target.
North End skipper Alan Browne and substitute Layton Stewart missed late chances to make it 4-0, but that did not matter as this game had been long since won thanks to a hugely impressive first-half effort from the in-form hosts.
