Yeah, that is a Linux thing:
It shows overall load, since you have 4 cores divide by 4.
Or, another way to look at it: 4.0 is 100% load.
The Pi, real world performance numbers show that it can route and SQM gigabits of packets using 25% of its CPU capacity or so.
Also, if you don't know about it Network/ interfaces/ Global network options you can use Packet Steering and watch the load become zero.
I, really do, suggest the Pi be the main router when you plan on upgrading.
Undervolting the CPU won't do as much as turning down the radios TX to only what is needed for stable throughput.
I'd swear I'd read the CPU controls the voltage but I can't find it and must have dreamt it.