EX 4200 10G interface speed issue

I am running Iperf between 2 servers conneted to EX 4200 via 10G DAC and i am only able to receive speed of upto 4.0Gbps any reason why i am not able to achive near desired speeds? 

Here's the iperf test result

Server listening on 5201
-----------------------------------------------------------
Accepted connection from 10.1.22.22, port 49768
[ 5] local 10.1.22.21 port 5201 connected to 10.1.22.22 port 49769
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-1.00 sec 456 MBytes 3.83 Gbits/sec
[ 5] 1.00-2.00 sec 459 MBytes 3.85 Gbits/sec
[ 5] 2.00-3.00 sec 461 MBytes 3.87 Gbits/sec
[ 5] 3.00-4.00 sec 465 MBytes 3.90 Gbits/sec
[ 5] 4.00-5.00 sec 462 MBytes 3.87 Gbits/sec
[ 5] 5.00-6.00 sec 460 MBytes 3.86 Gbits/sec
[ 5] 6.00-7.00 sec 461 MBytes 3.86 Gbits/sec
[ 5] 6.00-7.00 sec 461 MBytes 3.86 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 5] 0.00-7.00 sec 0.00 Bytes 0.00 bits/sec sender
[ 5] 0.00-7.00 sec 3.32 GBytes 4.08 Gbits/sec receiver