In William Shakespeare's Macbeth, would you consider Macduff a hero or a coward for his actions in Act IV? Some people view him as both. Explain...

There are very pertinent reasons why some would consider Macduff both a coward and a hero. To understand this perception, one needs to look at the circumstances in which he did what he did. The context provides greater clarity.

Firstly, one should understand that Macduff is very loyal to both his king and country. In Act 2, scene 3, it is he who discovers Duncan's most foul murder and when Macbeth later declares:

... Against the undivulged pretence I fight
Of treasonous malice.

Macduff retorts: 'And so do I'. This makes it quite clear that he will do everything in his power to fight the treasonous evil that has reared its ugly head in Scotland, just as he did when he fought against the traitors MacDonwald and the thane of Cawdor as well as Sweno and the Norwegian forces.

When Macbeth explains why he, in a moment of overwhelming passion, had killed Duncan's supposed murderers, one notes Macduff's skepticism, for he asks Macbeth why he did so. He is clearly not satisfied by Macbeth's explanation, for he later declares that he will not attend Macbeth's coronation, but would rather travel to Fife, his castle. He also makes the following statement Ross tells him that he will attend the coronation at Scone:

Well, may you see things well done there: adieu!
Lest our old robes sit easier than our new!

The fact that he does not attend the coronation and expresses his concern that their new leader (Macbeth) might be a greater discomfort than the previous one (Duncan) conveys his suspicion that Macbeth had a hand in king Duncan's murder.

Macduff disappears and we only hear of him in Act 4, scene 2, when Lady Macduff asks Ross why he had fled. Macduff's actions might seem cowardly in this regard, for he had left his wife and family unprotected, vulnerable to Macbeth's murderous spree. He seemingly was thinking only of himself when he hurriedly left Scotland.

Lady Macduff is deeply upset about her husband's actions and even when Ross asks her to not be as harsh in her judgment of him, she refuses to budge, seeing Macduff's actions as a betrayal of his family. She is so disturbed that she tells her son that his father is dead, not physically, but dead to them since he has abandoned them. This then, can be deemed as extreme cowardice. His entire family is later murdered by Macbeth's assassins.

We learn later however, that Macduff has gone to England to join Malcolm, the true heir to the Scottish throne (so named by his father, when he gave him the title, Prince of Cumberland) to raise an army against Macbeth. They have already received the support of king Edward and would march against Macbeth with troops led by Siward. Malcolm tests Macduff's loyalty by saying that he will be a far worse king than Macbeth could ever be and that Macduff should tell him if one such as he would be fit to rule Scotland. Macduff, in a moment of deep despair, passionately cries out:  

Fit to govern!
No, not to live. O nation miserable, ...

... Fare thee well!
These evils thou repeat'st upon thyself
Have banish'd me from Scotland. O my breast,
Thy hope ends here!

Macduff says that such a ruler should rather be killed. He would, instead, suffer self-imposed banishment than serve under such a corrupt ruler. He is clearly loyal to his country and refuses to serve a king that would lead it to its doom. He declares that his hope of setting things right in his beloved Scotland ends at this moment. He is clearly deeply distraught. Malcolm however, assures him that all he said has just been a test to see where Macduff's real loyalties lie.

The reason why Macduff fled Scotland now becomes pertinently clear. He wants to rid his country of the tyrant Macbeth as soon as possible. There was not a moment to lose. If he should have tarried, he might have been found by Macbeth's killers and been assassinated, just as Banquo and others had been. Macduff realised that he would leave his family vulnerable, but he sacrificed their safety for the greater good - the rescue of his beloved Scotland.

Macduff had made a desperate choice: Stay behind and ensure your family's safety and be killed in the process, or go to find help to destroy the malignant tyrant and ensure a life free of tyranny for you and your family. He possibly naively believed that the depth of Macbeth's evil would not be so great as to slaughter his entire family in their fragile state and that he would grant them some mercy.

In this sense then, Macduff's actions were heroic.