The firm said a curved 65in (165cm) set would go on sale in Europe, South Korea and North America in September.
It calls the move a "first" since other firms sell models that offer either one or the other technology, but not both.
But while LG described it as a "game changer", one expert said the move "did not make economic sense".
Sony and Panasonic showed off their own prototype 4K OLED (organic light-emitting diode) sets at the Consumer Electronics Show in Las Vegas in 2013, but have opted not to put them into production.
The two companies offer 4K sets based on synthetic LED tech instead.
The difference is that OLED makes use of a carbon-based compound that emits light when struck by an electric current.
This allows TVs to do away with a backlight, meaning that pixels can offer deeper blacks when they are not in use, allowing manufacturers to boast improved contrast ratios - similar to what used to be available via plasma screens before they were discontinued.
Samsung - the world's bestselling TV maker - did release two of its own OLED sets in 2013, but they both had 1080p resolution, offering about four times less definition than 4K. Its website says both are "no longer available".
Samsung does, however, offer a wide range of 4K models using synthetic LED panels.
In January, Samsung's visual display division chief, HS Kim, told USA Today that it was proving difficult to manufacture OLED TVs, which in turn was making them too expensive for the vast majority of consumers. He added that this was unlikely to change for "three to four years".LG describes the marriage of 4K and OLED as representing "the future of television"
LG's forthcoming set will cost 12m won ($11,765; ?7,095), which is about two to three times the price of existing 4K LED sets of a similar size.
That may limit its appeal, but the firm is focusing on what the launch represents, calling 4K OLED "a new paradigm".
"OLED TVs are expected to overtake LCD [Liquid Crystal Display] in sales within a few years and no company is better prepared for this than LG," added Hyun-hwoi Ha, president of the company's home entertainment division.'Lack of content'
One industry watcher agreed that LG's should offer best-in-class quality.
"OLED technology delivers the most vibrant and natural on-screen display that can be currently done with available technology," said Chris Green, principal technology analyst at the Davies Murphy Group consultancy.
"Combine that with 4K - which takes us to a new level of definition and sharpness - and you suddenly have a television that is pretty much like looking out of the window."
However, he added that the lack of 4K content coupled with the new set's price would prove a major deterrent.
"There's a lack of a compelling reason for you to go out to buy one tomorrow," Mr Green said.Existing 4K TVs may be less vibrant than LG's set, but they are sold at a fraction of the price
"And in most of the world the broadband isn't there to deliver a 4K stream to many people's TVs.
"Other firms aren't going to want to commit to tooling up a line to build a TV that they're not going to be able to shift in enough volume to make viable."
While the BBC and Sky Sports have internally tested 4K broadcasts, neither is ready to announce a date the facility will be extended to the public.
Sony does sell a 4K media player - which downloads films from the internet - but it has only been released in the US and requires a Sony TV.
Samsung sells a hard drive loaded with five 4K movies and three documentaries, but this costs $300.
Netflix offers a limited amount of ultra-high definition TV shows- including House of Cards and Breaking Bad - but notes that the service depends on subscribers having access to a 15 megabits per second internet connection.
Otherwise, 4K content is currently limited to downloading clips via YouTube and Vimeo or playing back material shot on high-end smartphones, digital cameras and camcorders.
LG has also announced plans to release a 77in (196cm) 4K OLED model at an unspecified date.
It added that it would show the new sets off at the IFA tech show in Berlin next week.