I have four tomato plants, three of which are together in one area and the last one by itself in another area. The three that are together have been looking sick recently. Their leaves used to be flat, but now they're kind of curling upwards. My other tomato plant that's by itself has perfectly healthy foliage and looks great, but these ones don't. When I shake the plants a little, I can see little white bugs flying from underneath the leaves, but there aren't that many. Should I be concerned for this leaf curl and these bugs?

Mix 2 tbsp of neem oil into a gallon of water and spray it on leaves covering both the sides of the leaf to get rid of the white flies.

Hello-
It is hard (or impossible) to diagnose a problem like leaf curl from a distance; but fortunately you can likely do it yourself!

Often when you see curling leaves (at least early in a plants growth) it indicates stress likely linked to watering...the problem is that the symptoms for under watering and over watering are identical.

Luckily nature has provided you with a highly accurate diagnostic tool; your index finger.

Simply stick your finger in the soil in your plants root zone and feel whether the soil is moist below the second digit of your finger. If it is; don't water the plant. If it is dry; do. And for tomatoes, as with most plants, the best method for watering is to give them a less frequent but much gentler and deeper watering, never from above and to mulch the soil deeply to conserve the water you've given them and prevent soil splash.
White flies? I am a lazy and cheap gardener - I don't want to be treating against insects that might/probably are not causing any damage. Just because a few insects have perched on your plant leaves does not mean they are causing harm and I would recommend against 'treating' them.

They likely have just stopped to admire your garden handiwork. Please wait until you see signs of damage or illness before deciding to take steps. Generally the best steps you can take are by creating good growing conditions for your plants, building the quality of soil in which you garden, and inviting diversity...let the good bugs fight it out with the bad ones.

All that said - I think your tomato plants look pretty healthy, now pinch out those suckers, remove a lower branch or two and research recipes for the food of the gods; tomato tart. Summer 'aint summer without tomato tart, I say.