Daniel,  
Hi there,  I hope you're well.  One of our S4048-ON switch has failed, we have a replacement and I need to get this added to the stack. Could anyone please advise whether or not the unit being added to the stack must have the same OS version to the master unit or whether that device is capable of downloading and installing from the master unit?  Can you please advise also whether the below steps are correct? If I manually assign the stack ID, power off the switch and connect the stack cables to the master unit will it boot and join the stack without rebooting the stack? All advise would be greatly appreciated. 
S4048-ON Adding New Unit to Existing Stack