This business about leftovers is so daft. I mean what can you really do to s plate of food (bar chewing it and spitting it out) that renders it inedible forevermore?
To be frank, in our house we do NOT waste food so if the 4yo doesn't eat what's put on her plate someone will - usually my DP who is a (irritatingly skinny!) bottomless pit. Is he being cruelly abused/cruelly abusing himself eating her leftover "dried out/congealed" leftovers? (FYI they're not, they're fine. We're talking meat/fish and vegetables or some sort of a stew/mince dish here, they don't go toxic if left covered over in the fridge a few hours - what the hell are people cooking that can't be eaten after being on a plate for that length of time?? I mean it's not like we'd reheat scrambled eggs 
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OP we're having the same issue with our nearly 4yo at the moment - nothing you're doing, it must be a phase for the age!
I do wonder if it's out schedule as so many people seem to feed their young kids dinner and get them to bed really early (eating at 5, asleep by 7) - but she's never been much of a sleeper, goes down at 8ish so we all sat dinner together at 6 - it's important to me that we eat together as a family and that we eat the same thing - roughly - we toddlerise a lot of our meals for her as she has a thing about "mixed food", so we'll be eating say a veggie pasta bake and she'll have the veggies roasted separately, plain pasta and the sauce and cheese on the side in little bowls - you can see what a cruel hardarsed mummy I am! 
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Her thing is she will almost always eat her carbs first then claim she's "full" and start arsing around - making a racket with her cutlery, jumping up and down from the table for cuddles, going off to the playroom and bringing toys back to the table, crawling around under the table etc. Bloody annoying!
We've tried various things around this, but these things have been the most effective (like most people we're only human so can only be 100% consistent on a good day!):
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do not allow her to disrupt our meal. I tell her if she's done eating and doesn't want to sit nicely and talk to us she can go to play in the playroom (next door). She hates that because what she's after is attention, which I'll happily give her if she sits with us and talks to us but I'm not going to stop eating before in finished to dick around on the floor with her. So she'll usually at least come back and sit at the table for a chat, which opens the door to her potentially finishing her meal. The flip side of this is that if she's at the table eating and/or behaving nicely, she gets our full and undivided attention. I personally like flipping through a cook book at the table at mealtimes, but soon realised the nights I was doing this her behaviour was far worse - which made me aware I was actually being quite rude, using our family time to amuse myself. So I've stopped.
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smaller carb portions/splitting her carb option in two and giving her half at the beginning of the meal and half at the end.
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no more snacks in the day except fruit/veg sticks. She'll by and large refuse this option so is pretty much on three meals a day now. If she eats all her lunch (which is a much less problematic meal usually) she is offered a sweet treat (yoghurt, biscuit, small piece of cake or similar) and will usually snap this up. But between meals, it's fruit/veg or nothing. We've been trying this for a week or so now and dinner time is much easier.
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I never ever make/bribe her to eat anything. Because it's attention, and that's what she wants. She is entitled to control her own intake. But I am her mother and have to make sure that a good proportion of what she does eat is nutritious, not just empty carbs to fill her up. We've talked about protein, veggies, carbs and fats, and how they are all important and do different things for her body but she needs to eat some of all of them to grow up strong. She knows which foods fall into which groups. She knows sweet things are treat things. We'll get there in the end!
If all the above fails, she doesn't eat her dinner and then claims to be hungry later, I will either offer her her leftovers again (awful awful parent 😆) or if she genuinely doesn't like it (i.e. she has to have tried it) or her dad has already snaffled it, she can have the dreaded banana. She usually opts for hunger. If she's barely eaten anything all day then I might unbend and give her a bit of toast, but as others have said this is her go to favourite food and not particularly nutritious or filling, so I try not to or she'd never eat anything else. I refuse to feel bad about her going to bed hungry if she's been presented with perfectly acceptable options - that's not taking her power away, it's giving her choices, including the option to make what I think to be a bad choice.
She's just trying to exert a little power, which I see no need to stamp on but she needs to understand that the power to make her own choices does not equate to power over me or her dad.
Hope we both get there OP! The main thing is to take the emotion out of it - it strikes us at the heart as parents as we feel so responsible to make sure they are well fed with good food - but as others have said, connecting food with feelings, particularly feelings of obligation/guilt/shame/comfort, is a really bad idea as I know to my cost. It's just food. To be taken or left. Manners around mealtimes are more important (for all of the family!)