Obviously, yes. 4 oz would be "very heavy." For field use that is. For a hike, who cares?
Yep. When I do ranch work, I have a big multi-tool, maybe a sledge hammer, spool wire, pliers, pockets full of etc. But like you said, you're only carrying it from the pickup to the fenceline, and then if you have to go back to the pickup you're not carrying anything and really that walk is a break not a chore. Then at the end of the day you carry the shit back to the pickup - once. Not even close to the same as what I'm talking about.
If that's true then more power too you (and your poor knees!) but I have to call it into question. An above-average hiker is making 2mph. I do more than that but I'm experienced. So let's say you're doing 2mph. That would be 4-5 hours of walking if you walk 8-10 miles per day. That means that you've only got 3-4 hours for work (assuming an 8 hour day).
If you're really working for someone who lets you work 3-4 hours per day, all I can say is... are they hiring?