I have a fiber optic network of one server connected directly to 4 clients(they are not connected to each other)
I'm assuming you're not using a switch.
The network is suppose to support up to 4x10Gbs, so 10Gbs throughput from each client to the server. Given the server connected directly to 4 clients and the network has the proper hardware and is setup properly, am I suppose to expect a nearly full throughput on almost every throughput test?
Yes - provided each node can keep up with the speed and whatever you're transmitting can be provided or stored fast enough. If there's storage access (e.g. file server) the storage needs to be able to supply data at that rate.
Additionally, the node needs to be able to process the data rate from/to the NIC. In extreme, the server's X710 NIC requires 5 GB/s slot bandwidth, so the slot would need to be PCIe 3.0 x8. A PCIe 2.0 x8 or a PCIe 3.0 x4 slot limits the bandwidth to 4 GB/s, PCIe 2.0 x4 to 2 GB/s and so on.
Depending on the processing required, the CPU can also be a bottleneck.
I have an issue with software/network
You're not telling us which issue.
and I wish to tell that given a test of my network with 3rd party tool such as iPerf, what throughput am I suppose to see?
Assuming sufficient bandwidth for the NIC slots, iperf should show very close to 10 Gbit/s throughput for each connection and 40 Gbit/s in total. Effective TCP throughput should be close to 1.18 GB/s per link.
I am not sure what should I expect from fiber network and how stable it's suppose to be?
Fiber or twisted-pair doesn't matter as long as the cable is fine. Ethernet usually aims for a bit error rate to 10^-12 or better, so the frame error rate should be lower than 1 in 800 million. If the error rate is higher there's a problem with the cable (type, length, damage, dirt, ...).
Fiber is sensitive to sharp bends, single-mode even more than multi-mode. Make sure you observe minimum bend radius.