By Gabriel TanFollow @@gabetan13
The reigning Premier League champions got off to a horror start at the PTT Stadium when Kirati Keawsombat fired the hosts ahead after just four minutes, and it was 2-0 in the 35th minute after Ivorian forward Baba Ouattara found the back of the net.
On the stroke of halftime, Kirati doubled his tally for the night to put the Oil Millionaires three goals up and it looked as though they were all but through to the last four.
But four minutes after the hour mark, Muangthong pulled one back through a strike from Teerasil Dangda and three minutes later, a stunning comeback was on the cards when Edivaldo Rojas scored to make it 3-2.
And with six minutes left on the clock, Teerasil struck again to send the game into extra-time, where the Twin Qilins eventually prevailed after Monkol Namuad netted what proved to be the winner in the 112nd minute.
Defending champions Buriram United are also progressed from the quarter-finals after cruising to a 3-0 victory over Division 1 League outfit Singhtarua.
Jirawat Makarom fired the Thunder Castle in front three minutes after the half-hour mark, before Carmelo Gonzalez added a second just before halftime to put them in a commanding position.
But just for good measure, captain Suchao Nutnum weighed in with a third in the 78th minute to cap off a fine evening for the hosts.
Over at the 72-years Anniversary Stadium, Bangkok Glass were made to work before recording a 1-0 win over Bangkok FC.
The only goal of the game came in the 27th minute when Hironori Saruta found the back of the net, keeping the Glass Rabbits' hopes of winning silverware this season alive.
Finally, completing an all-Premier League final four were Police United, who saw off Chiangrai United 3-1 on penalties after both sides played out a 0-0 draw.
Wednesday's Thai FA Cup Quarter-final Results
Bangkok FC 0-1 Bangkok Glass
Buriram United 3-0 Singhtarua
PTT Rayong 3-4 Muangthong United (after extra-time)
Police United 0-0 Chiangrai United (after extra-time, 3-1 on penalties)
Image courtesy of Muangthong United FC