First and foremost, "sturdy" matters more than "light" - in fact, a "standard design goal" for a standard well cover is that you should need at least two people to remove it - helps prevent Timmy from popping it off so Lassie can go get someone to rescue the idiot boy again when he falls in the well, again. That poor dog...
Likewise, if a fat person or whatever your largest local wildlife is (so if your bears are Grizzlies rather than Black Bears you need to plan for more weight) decides to climb up on the cover for a look around, it needs not to collapse and dump them in the well. Nothing like rotting meat to spoil your water supply.
So, if going for a "flat" roof (please actually give it at least 1/4" per foot slope) you want something like a small deck. It's only 4 foot span (or perhaps 2 foot 8 inches if 4 feet is the outside dimension and it's 8" block) and "it's not a deck, so it doesn't have to meet actual deck codes" but it does have to hold up a person, bear, whatever. So you can use 2x4 framing, or perhaps skip framing if it's only a 32" square hole, but at least choose thick enough exterior or marine-grade plywood to unquestionably support your largest load with some safety factor.
An alternative is to use a "non-flat roof" as they are easier to waterproof effectively, and may be less attractive as a climbing surface. If 4 feet is the outside dimension, a "5/12 pitch roof" only needs to be raised 10 inches in the middle (2 feet of run each way), and will shed water far more effectively than a flat roof.
You can also go all the way to a "well house" built on top using the well as a foundation, but I suspect that's not a direction you are interested in.
Any well cover, particularly if it's "tight" (as it should be to keep critters out) is normally vented (with screened vents to keep critters out) so that air can get in as you pump water out.