It is possible to convert a QSFP or QSFP+ port to an single SFP or SFP+ port using the Quad to Small Form Factor Pluggable Adapter (QSA). However, in order for this to work, you'll need to configure the QSFP+ port into break-out mode to 4x10Gbps, similar like you would when using actual QSFP to 4xSFP break-out cables.

For OS10, the CLI for this action is:

interface breakout 1/1/7 map 10g-4x

This will break out the interface 1/1/7/ to 4 10Gbps links, which translates to these interfaces on the switch: 1/1/7:1, 1/1/7:2, 1/1/7:3 and 1/1/7:4:

show interface status
                              
------------------------------------------------------------------
Port       Description  Status Speed Duplex Mode Vlan Tagged-Vlans
------------------------------------------------------------------
Eth 1/1/1               down   0     auto    -
Eth 1/1/2               down   0     auto    A    1    -
Eth 1/1/7:1             down   0     auto    A    1    -
Eth 1/1/7:2             down   0     auto    A    1    -
Eth 1/1/7:3             down   0     auto    A    1    -
Eth 1/1/7:4             down   0     auto    A    1    -
Eth 1/1/25              down   0     auto    A    1    -

Only the first, 1/1/7:1 is usable when using a QSA. The other three will remain in link down state and configuring these ports will have no effect. Place the configuration of the port on 1/1/7:1:

show running-configuration interface ethernet 1/1/7:1
interface ethernet1/1/7:1
 no shutdown
 switchport access vlan 1
 flowcontrol receive on

I've also noticed that when linking up to a S5100 series switch, you'll also need to explicitly configure the port to a fixed speed of 10Gbps, as it will otherwise not negotiate properly when connecting it to a S4000 or S4100 series switch.