The 40,000 mark was not passed in 2015 - when the record Pro12 attendance figure of 53,000 was ultimately achieved - until the final two weeks of sale which accounted for 20,000 tickets, including a large 'walk-up' crowd of over 5,000 fans on event day.
The big double derby will be the grand finale to an unprecedented run of six Welsh derby games in a row - kicking off on Saturday March 26th when the Ospreys host Scarlets at the Liberty Stadium.
The fourth incarnation of the event, which sees the Cardiff Blues take on the Ospreys (2.30pm) and the Dragons take on the Scarlets (5.00pm) in back-to-back matches, has been boldly backed by WRU chairman Gareth Davies to fill the national ground to its 74,500 capacity.
And current ticket sales figures indicate that the chairman's prophecy is set to come true as fans snap up tickets to see their international heroes meet each other at Wales' national ground in regional colours.
"The response from supporters to Judgement Day has grown year on year and the event has become a must-visit part of the annual rugby calendar," said Davies.
"We want to fill Principality Stadium to its rafters for this true celebration of our national game. All three of our home Six Nations matches sold-out in record time this season and Judgement Day is a fourth opportunity for fans to see many of their heroes competing in an elite environment under the same roof.
"We will be adding the carnival mood of two top class regional fixtures to the kind of atmosphere you can only normally get at a home Wales international and throwing a little local Welsh rivalry to the mix.
"These matches have fast become 'must-see' games, the Judgment Day event is a true celebration of regional and Welsh rugby and we are delighted that it has once again captured the public imagination."
Mark Davies, Chief Executive of Pro Rugby Wales said: “The next six weeks in rugby is one of the most exciting and significant periods we’ve ever had in a block for professional rugby in Wales.
“Hot on the heels of a high profile and thrilling Six Nations campaign, rugby supporters will enjoy an unprecedented series of six big Welsh derbies in a row - which are all absolutely critical at this stage in the Guinness PRO12 battle for each of our four teams.
“Welsh derbies capture the true spirit and passion of Welsh Rugby – we saw positive success over Christmas and New Year with sell-out fixtures which shows how much our supporters enjoy these games.
“We’re expecting good crowds again at our home grounds for these exciting games culminating in the big double-header at Principality Stadium at the end of April when we’ll see all the best players in Wales - current internationals and future stars - playing on the same pitch on the same day.
“This is a great time to be a rugby fan and a huge opportunity for supporters in Wales to get behind their teams for these six big derby games - we’d hope this unprecedented run of key derby games will attract good travelling support from our most loyal supporters as well.”
Last year’s Judgement Day saw the Scarlets climb back into the Pro12's top six as they beat Newport Gwent Dragons 10-29 with a clinical display in front of the record crowd and Ospreys secured their place in the Pro12 play-offs with a convincing 23-31 win over Cardiff Blues.
Cardiff Blues are the 'home' side in the first fixture of the day on the final weekend in April, facing the visiting Ospreys, with the Newport Gwent Dragons hosting the Scarlets for the second clash.
Tickets for Judgement Day IV have been frozen at 2015 prices and can be purchased now at £10 per person (booking fees apply) from www.wru.wales/tickets, by calling 0844 847 1881, in person from the WRU Ticket Office on Westgate Street, from each of the four regions or from Tesco stores throughout Wales.
Season tickets purchased from either of the home teams - Blues and Dragons - include access to Judgement Day IV, with fans asked to liaise with the respective box offices.
Both matches will be shown live on television with BBC Wales showing the first match and S4C the second.
