Low GPU utilization

[MeeLee]

Your Celeron, dual core at almost 3Ghz is good enough to feed 3 GPUs of GTX or equivalent.
With that motherboard, your upgrade path is limited.
I guess you could get a Pentium Gold G5000 series CPU, which is dual core, 4 threads.
Preferably a 65W model, if your cooling is sufficient.
The cheapest one is a G5400, the most expensive one is a G5620.
I would choose the 5620 if you plan on upgrading GPUs, as they all will run 4 GPUs fine, but the lower end model might need more performance to push the higher end GTX 1660 series of GPUs, while the highest model might just be able to push 4 of them.

[scm2000]

Just to update.. I upgraded the CPU, now all 4 GPUs are being used at 100% each... This completes the build of my system.

[toTOW]

Are you sure that you have enough PCIe lanes to feed all those GPUs ?