From the perspective of a cyclist, what is a common way to hydrate throughout the day?

I often stop at gas station convenience stores when riding to just purchase a drink, finish it, and keep going on. Most places stateside those are open during corona shutdowns. In other years, the time-tested coffee shop or breweries.

I'll focus on this part of the question:

After becoming dehydrated, would it be best to rehydrate with water, or use some special kind of energy drink?

Normally, I don't recommend energy drinks simply to rehydrate.

Most drinks with electrolytes do contain carbs, but in theory that's a separate issue (e.g. Nuun tablets contain electrolytes but minimal carbs). Some simple carbs may (NB: not a nutritionist) help you absorb electrolytes. If you deplete your electrolytes, your power can drop off. Generally, a craving for salty foods will let you know that you've had your electrolytes depleted.

Also, cycling can burn a lot of carbs. If you cycle slowly for a long distance, you are burning a mix of carbs and fat, but you do burn a lot of total calories. If you are cycling intensely for long distance, you burn a ridiculous amount of total calories, the majority of which are carbs. Replenishing those enables you to ride further and, if you desire, faster. However, outside of cycling, I think it's a good general principle that we should limit the amount of processed foods, especially processed sugar. Thus, outside of a ride, I personally stick to drinking plain water to rehydrate. If I need supplemental electrolytes, I get that from my food - keeping in mind that things like frozen dinners and lunch meats have ample sodium already.

I used to stick to plain water even in hot weather. I persistently had mild headaches through the day, until I increased my electrolyte intake during rides and, in my case, my salt intake throughout the day (NB: I believe my sweat is fairly salty, but even if I'm wrong, I ride quite far and fairly intensely). Given the climate in England at this time of the year, though, this might not be the case for you.

I should add that I'm probably in a calorie deficit due to eating 2 small meals per day rather than my usual 3-4 medium to large meals per day. I've experienced this issue in the past after weight training on an empty stomach. A bottle of Lucozade and lying down for a couple of minutes fixed the issue. If the issue has nothing to do with dehydration, then it could be something to do with me not eating enough.

It sounds like you are on a diet. In addition to not being a nutritionist, I'm not even personally familiar with dieting. Normally, a 4 mile ride isn't that strenuous, but you are a new rider, and you experienced this while weight training as well. In a very general sense, it does sound like you may not be eating enough. Again, you do burn carbohydrates while exercising. You may not burn as much calories while weight training, but you do burn some, plus you do want to ensure your muscles get enough protein to repair themselves from that activity. You might want to re-examine your diet while exercising. I am not familiar with how much of a calorie deficit is safe while exercising. I do know that if you want to cycle a lot, you can't run a large calorie deficit.

Your symptoms don't sound like anything to do with dehydration. The first and most reliable sign of dehydration is intense thirst. It's a myth that "thirst is too late" or that you need to drink before you're thirsty.

I'm just not very disciplined when it comes to sufficiently fuelling my body.

This usage of "fuelling" is advertising terminology invented by the people who want to sell you Gatorade and Gu. The calories you used for this ride were in the form of glycogen stored in your liver and your leg muscles, probably earlier in the day or even the day before.

If you're hungry, eat. But the food you eat generally takes many hours to be converted into glycogen.

Now you're pre-warned with some of the symptoms and results, consider doing the same ride again and see how you go.

Perhaps pace yourself a little better on the uphills, carry a full water bottle and sip from it periodically. Stick a banana, a sweet thing, or a gel in your pocket for just-in-case.

Personally I feel ratty riding if I'm low on sleep/tired too. Make sure you're getting enough quality rest.

And do take a rest day occasionally. We're not built to do the same work day after day, a rest day is beneficial.

Monitor urine color. (pee in an empty clear bottle) If darker than light apple juice, drink more water. Brown apple juice is too dark.

I drink 2 to 3 ounces of water per mile. Maybe 0.5 to 1 ounces of Gatorade type drink per mile - on the high end on hot days. I sweat most of that out on 100 mile bike rides. I rarely stop to pee.

I have tried almost everything for food. Chocolate is always a mess. I rode 200 miles on peanut butter and jelly sandwiches. Felt great. My favorites are bags of cookies. I did 100 miles on two bags of cookies. Better than Clif bars.

30+ years of long distance riding