The most important thing about facing any essay question is to make sure that you have a strong argument in reponse to this. But what does that actually mean? Well, usually it means make sure you're saying something that answers the question. Engage with it. Instead of just writing a load of Latin down, take two or three minutes to think about the question. What's it asking? Bascially, do you like Dido? And why?
My answer to this question is 'sort of'. Not the most helpful answer in conversation, but certainly in an essay. BEFORE I WRITE ANYTHING, I've got to have a good think about what I'm actually going to say. True, I'll write less than if I started writing immediately; but a well structured, cohesive and substantiated answer is worth far more than random jottings. Again, what does that mean in English? It really means that you've got to make sure that your answer has some kind of flow to it - make sure all the paragraphs link up so that someone reading understands how you've connected things. 'Cohesive' simply means that you've considered both sides of the argument in your essay; and 'substianted' means that it's supported by evidence. YOU CAN NEVER QUOTE TOO MUCH LATIN, so make sure there's as much as possible in your answer.
If I leave space for the intrudoction (I find it best to do those later), I can begin with thinking about Dido sympathetically. My first paragraph could be about how the Book begins from Dido's perspetive - the first two words are 'at regina' (but the Queen), which anchors the narrative to Dido. This shift in protagonist from the narration of Aeneas in the previous three books presents her sympathetically as we (as an audience) are beginning to see the situation from Dido's perspective. Moreover, the lengthy speech she gives to her sister, Anna, illuminates her thinking much more clearly.
The second paragraph would point out that, despite this shift of focus that naturally changes our sympathies, the audience naturally feels sympathetic towards Dido because her situation is so awful. She has been forced out of her homeland of Tyre after the murder of her husband Sychaeus by her brother, Pygmaelion. Landing in northern Africa, she then fought to gain her own plot of land against the fierce local ruler Iarbas, who wanted to marry her, and is trying to make the best of it for her new city of Carthage. When faced with such difficulties, it is hard for any reader not to find a character who has faced so much yet still endures and fights with spirit sympathetic.
The next paragraph should point out that, despite this inital sympathetic presentation, Virgil undercuts this image with some very unsympathetic moments. Dido, for example, enters the temples of the Gods at one point - something that was considered sacriligious to most Romans. It is worth bearing in mind that state religion was a much bigger deal in Rome than modern society: they even had a word ('nefas') for acts that would have angered the gods. In addition, Dido acts in a way that is highly unladylike for Roman society, the epitome of which is when she has sex with Aeneas in a cave while they're hunting without any formal promise of marraige. Romans would have viewed this very badly - consider the ancient laws on adultery (e.g. 'lex Iulia de adulteriis') and some of Ciero's opinions on female sexuality (e.g. the presentation of Clodia in 'Pro Caelio').
The final paragraph could focus on Dido's descent into madness throughout the book. She wanders around Carthage at night and even 'fondles' Ascanius - Aeneas's son - while thinking about Aeneas as a way of dealing with her desires. This is before the full crazy is released, when she builds herself a funerly pyre, which, after she's set it alight, she forces Aeneas to watch her stab herself with his sword and then jump onto the fire. Simply because he's been told by Mercury he needs to travel to Italy now. Not the best reaction to a break-up the world's ever seen.
My conclusion would focus on how Virgil skilfully manipulates his readership into feeling sympathy for Dido in the early stages of the book, while placing the doubt in their mind as to her character before revealing how unhinged she is. At this point, I think most readers lose sympathy for the character, so we are forced to conclude that Dido is ultimately an unsmypathetic character within Aeneid 4.
REMEMBER TO CHECK YOUR WORK - you only need to leave 5 minutes at the end for this. It's amazing how many marks you'll lose for spelling, grammar and translation which are only silly errors. By making sure these things are accurate you'll have saved lots of marks that you should get anyway.
As I said, the most important aspect of any essay is making sure it goes somewhere - answer the question! It doesn't harm to reuse the words from the title to show the examiner you're really tackling the question and making sure you give a full answer.
I hope this helps!