4-Pin Molex to PCI-E Power is not a good idea, even if you think that everything should be fine with the cables you have available and make sure you do not overload the power lines on the PSU... then suddenly the connectors on the cables start failing you.

More up to date yet similar problem are the SATA power to Molex power adapters, though there is a solution for that as well in order to minimize the possible issues: http://cryptomining-blog.com/8792-dual-sata-power-to-4-pin-molex-power-adapter-cable/