You can run a 1/8 buggy on 3s. I've run my truggy on 3s and no problems with heat or anything. You just have to make sure your setup can handle it and the batteries can provide enough current. Not ideal...just saying it can be done. I think a 1/8 buggy would run fine on 3s...they're a lot lighter than truggies still.
Compare the sizes of the arms, bulkheads, diffs, and shock towers of 1/8 buggies to 1/10 buggies and even truggies, and you'll see where the weight comes from. It all adds up. Durability...well, the more weight there is, the stronger the parts have to be to take the impact of that additional weight coming to a stop. These days, 1/10 trucks are pretty durable. The parts are smaller, but there's also less weight resulting in less energy to absorb.
If you want to see how fast a sub-5lb 1/8 buggy is, wait until I get my project done. I'm converting a 1/10 Kyosho DBX via a modified chassis to 1/8 buggy size. It's already the same width if I use 1/8 buggy wheels/tires (it's actually 1" wider if I use the stock 2.2" wheels/tires). It's 20mm shorter in wheelbase than most 1/8 buggies. I'm resolving that with a new chassis. It will weigh a tad over 4lbs when done, running a 3300kV Medusa 3650 motor on 3s. It's stupid-fast right now. And I expect it to be a bit easier to control with the longer wheelbase when I'm done with it.
you are dreaming if you think you can make a 4 lb 1/8 size 4wd buggy using 1/8 tires. that is assuming you plan on weighing it track ready with battery body ect. even with the most expensive fancy materials it would be difficult and very expensive to make a reliable sub 7 lb buggy with a reasonable size battery.