The latest announcement comes only three months after Uniper said the plant's commercial operation would be delayed until the final quarter of this year, having previously planned to start it in the first half.
"The currently planned commercial operation date is not before the fourth quarter 2018. Given pending lawsuits further delays cannot be ruled out," the group said in its annual report published on Thursday.
The construction of Datteln 4 has so far swallowed significantly more than 1 billion euros (892 million pounds) in investment and more is needed to complete a site that was originally supposed to start up in 2011.
Uniper last year filed a lawsuit against rival RWE for cancelling a contract to buy power from the plant after continued delays and a significant decline in wholesale power prices over the past decade.
Uniper, in which Finland's Fortum agreed to take a 47 percent stake last month, said that there was a risk that all investments made into Datteln 4 will have to be written off if a new development plan was declared invalid.
The company's full-year results on Thursday showed an 18 percent decline in operating profit, with Uniper citing weakness at its gas optimisation business.
Full-year adjusted earnings before interest and tax (EBIT) came in at 1.11 billion euros, slightly lower than the 1.16 billion average forecast in a Reuters poll.
(Reporting by Christoph Steitz and Tom Kaeckenhoff; Editing by David Goodman)