If Miley Cyrus says "We Can't Stop," Sinead O'Connor is unstoppable too. The "Nothing Compares 2 U" singer is serious about her apology from the 20-year-old pop star and has penned a fourth letter to her in which she slammed Cyrus for "stigmatizing mental illness" and asked her to apologize.

The 46-year-old singer criticized Cyrus' response to the initial letter, arguing that Cyrus' insensitive remarks about her made O'Connor a victim of cyber bullying. She demands Miley's apology to Amanda Bynes and all the people who suffer from mental health disorders as Miley's remarks have been taken in a negative way by people who are sending her hate mails. Read the entire letter here

"No person, myself included, should wake to the types of emails and communications I have had for the last few days urging that I should kill myself," she wrote. "Nor to the type of Net abuse I have had to endure as a result of what you did."

"I might add my children have been extremely upset by the nature of media attitudes toward their mother on the grounds their mother is (falsely) believed to be mentally ill," O'Connor writes to Miley in her latest open letter.

Miley's posts, according to O'Connor, have damaged her reputation in the music industry as she has to now explain her position to everyone that she is not suffering from any mental problems.

"Neither should I now have to have management write to half the show-business world to explain my tweets were two years old and that ... I am very fit for work," she continued. "I have four children to support. I can't do that if people believe me to be unemployable."

The Irish singer also requested an apology to "all sufferers of mental health difficulties and all those who have had experience of suicidal feelings or who had been affected by suicide."

The matter started when the Miley said in an interview some time back that she wanted the music video for her single "Wrecking Ball" to be similar to the famous hit "Nothing Compares 2 U."

Sinead took to her blog suggesting Cyrus is allowing herself to be "pimped" by the music industry. The veteran artist also expressed her concern that those around Miley have led her to believe "that it is in any way 'cool' to be naked and licking sledgehammers" in her video.

O'Connor's first letter then prompted Cyrus to post a series of Sinead's tweets where she sought help from a medical person and compare her to Amanda Bynes. This prompted Sinead to pen a second letter to Miley, calling the performer "anti-female" and "f--king stupid."

"Sinead. I don't have time to write you an open letter cause I'm hosting & performing on SNL this week. So if you'd like to meet up and talk lemme know in your next letter. :)," Miley replied on Twitter.

The reply from Sinead was not unexpected. "You can take five minutes today between g- string f--kin' changes to publicly apologize and remove your abusive tweets," she wrote. "When you publicly apologize to Amanda and myself and all mental health sufferers as well as all who were abused by priests that will end the matter as far as I am concerned...When you end up in the psych ward or rehab I'll be happy to visit you.. and would not lower myself to mock you."

Miley was asked about Sinead's letters during her appearance on the Today show to which she said that she respects Sinead as an artist and does not mind her letters.

"I think she's [Sinéad O'Connor's] an incredible artist, I think she's an awesome songwriter, and I was really inspired by her for my "Wrecking Ball" video, which was what started the whole thing," Miley told Matt Lauer.

"I don't know how someone can start a fight with somebody who said, 'Hey, I really respect you. And I really love what you what you did.' 'You know what? You suck! I don't like you!' That was kind of crazy. But as I said, I'm a big fan of hers, so it doesn't really matter."

"It's all good. You can write as many open letters as you want," she added. "That's really what blogging is. I get open letters every day. It's nothing too new for me."

Miley is behaving how a 20-year-old is expected to - stubborn, unapologetic at times and following her heart. Miley respects Sinead and she has made it clear that she is not bothered by criticism from anyone. Should the veteran artist like Sinead then get a hint and put an end to the matter?

"Why argue with a 20 year old. Be wiser. She is simply a dumb child," Christine Stanton a fan of Sinead wrote on her Facebook page.

"I have to say... this has grown very old. Don't embarrass yourself by hanging on to it," writes another user Leslie Jean-Jellybean on Facebook.

Miley is just being Miley. Do you think the wild at heart artist will apologise to the very mature Sinead O'Connor?