Malcolm pretends to be even more evil than Macbeth. Why does he do this? What does he discover by doing so?
When Malcolm tells Macbeth that he lacks all graces including "justice, verity, temp'rance, stableness, / Bounty, perseverance, mercy, lowliness, / Devotion, patience, courage, [and] fortitude," he is, thankfully, pretending (4.3.108-110). However, each time he tells Macduff about one of his terrible qualities, Macduff assures him that such a quality can be overlooked, especially when compared to the tyrannical Macbeth.
When Malcolm says that he is lusty and lascivious, Macduff tells him, "We have willing dames enough" (4.3.87). When Malcolm tells Macduff that he is greedy and power-hungry, Macduff assures him, again, saying that "Scotland hath foisons to fill up your will / Of your mere own" (4.3.104-105). He tells Malcolm that these faults are supportable when weighed against other graces that Malcolm surely possesses. In doing so, Macduff inadvertently makes Malcolm suspicious that he's been sent by Macbeth to lure him home to Scotland so that Macbeth can have him killed.
When Malcolm presents himself as so fully terrible and unredeemable, he essentially tells Macduff that he will come back to Scotland if he seems fit to rule. Macduff responds most desperately, saying, "Fit to govern? / No, not to live" (4.3.120-121). He gives up trying to convince Malcolm because he now believes that Malcolm's rule would be as bad as (or worse than) Macbeth's. At this point, Malcolm can be honest; he'd been testing Macduff because, he says,
By many of these trains hath sought to win me
Into his power, and modest wisdom plucks me
From overcredulous haste. (4.3.136-139)
Macbeth has, apparently, sent others to Malcolm to try to lure him home, and so Malcolm has become wary and distrustful of such efforts. By conducting this test, he's discovered that Macduff is honest and does want the best for Scotland (not to have Malcolm killed). Now, he says that he's never lied until now, he's never slept with a women, and that he is ready to take his place on the throne and do everything he can to help his country.
Malcolm's father, the late King Duncan, had a fatal flaw and that is he was too trusting -- and paid for it with his life and crown. Malcolm is not so trusting -- he pretends to be full of vice to Macduff when Macduff joins him and the English army to gather forces against Macbeth. Malcolm is testing Macduff's loyalty in a way that Duncan should have tested Macbeth's. Malcolm tells Macduff that he has been with many, many women, drank, gambled, lied, swore, and all that fun stuff -- all to gauge the effect his words have on Macduff. When Macduff turns away in sadness from these words, Malcolm has his answer -- Macduff is loyal and the one to recruit to dethrone Macbeth. Malcolm is much more canny than his unfortunate father.
Malcolm declares to Macduff that he is wicked because he has excessive lust and greed and in fact has no redeeming qualities to offset these faults. He is testing Macduff's loyalty to Scotland because he does not know Macduff well and is unsure of Macduff's motives for coming to England. Is Macduff there because he seeks some personal gain, or is he interested in the good of the country? By pretending to be more evil than Macbeth, Malcolm discovers that Macduff really is more concerned about the country than he is about anything else. Therefore, Malcolm decides he will return to England with an army to overthrown Macbeth, just as Macduff had hoped.