The reluctant witness codenamed Person Four came under intense cross-examination on his fourth day in the Federal Court witness box, after alleging Australia's most decorated soldier kicked a handcuffed Afghan prisoner off a cliff.

The incident in the village of Darwan, Uruzgan province in September 2012, occurred before the prisoner was shot dead near a creek bed below.

"I kicked the c*** off the cliff," Person Four alleges Mr Roberts-Smith said.

Person Four says Mr Roberts-Smith covered up the execution and directed his troop to lie, but later heard the former SAS corporal repeat the story.

It's a claim the former soldier denies, along with a string of allegations published by media outlets which have led to the defamation law suit.

Mr Roberts-Smith's barrister Arthur Moses SC asked why someone of his client's stature would publicly boast about such an action.

"(Mr) Moses you seem to forget he was a VC winner at this time, he was running by his own narrative," Person Four said.

"The only one running by his own narrative is you," Mr Moses responded.

Person Four: "Absolutely not ... there are questions being posed to me here that I've never heard before".

"That's because your evidence is being tested isn't it?" Mr Moses replied.

"Test all you want," the witness said.

The former elite soldier also spoke of "obsessing about injustices" he felt following the 2010 Battle of Tizak in Afghanistan, for which Mr Roberts-Smith was awarded the prestigious Victoria Cross.

"The reason why I obsess ... the regiment doesn't deserve liars and cowards. Your Honour I'm not a liar or a coward," he said.

He clarified such labels did not apply to Mr Roberts-Smith, who he had seen perform heroic feats on the battlefield.

Person Four earlier testified "both of us did as much as each other that day," and felt hurt he was awarded a medal of gallantry two years after his comrade.

He was asked if other soldiers were using his personal upset at the awards process to attack the decorated soldier.

"That's how I felt. I believe I've been manipulated," he said.

Mr Roberts-Smith, 43, is suing The Age, The Sydney Morning Herald and The Canberra Times over reports that he committed war crimes and murders in Afghanistan between 2006 and 2012.

One of a handful of Australian recipients of the VC since 1970, he has suggested some claims stem from jealous associates spiteful of his medallic achievements.

Person Four agreed that Person Six hated Mr Roberts-Smith with a passion, and wanted to "bring him down".

He suspected another soldier had divulged to journalists a private conversation about an alleged war crime. This act had upset him, he said.

The former special forces soldier said after Mr Roberts-Smith and another patrol commander left their army squadron, the situation intensified.

"The sharks were circling, put it that way," he said.

Person Four - who was medically discharged from the army last year - said he spent some time in a mental health facility because of this trial and his angst about giving evidence.

He was formally excused from giving evidence about a 2009 mission dubbed Whiskey 108 in which the trooper on his first deployment "outside the wire" was allegedly ordered to shoot a prisoner.

Person Four who objected on the grounds of self-incrimination, couldn't be certain that he wouldn't face questions about his alleged execution, but understood there may have been an agreement between his lawyers and the news outlets.

The trial before Justice Anthony Besanko continues.

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