;)Hope it can work!

Is your TL-PA9020 UK version or EU version? UK version should upgrade UK firmware. Suggest you to update to one published on 2016-10-25 first. Remember to extract it first.

Did you add some electrical devices recently? It may be caused by the interference. Move the powerline adapters away from other electrical device and power adapters, and plug them both into the power...

140208 is not in the firmware list for TL-PA4010. It seems to be a special firmware or the model number is not right. You can ask TP-Link support whether there is some special way to upgrade it.

I got a PLC firmware recovery tool from TP-Link here: http://support.tp-link.com/f/060a1dd7d0/?dl=1 Suggest you to update to the last firmware....

No difference. Both adapters in a kit can be either sender or receiver. 2 kits have 4 adapters, enough for your need.