when ya run fo4 it should launch a loader first .. try messing with the gfx settings maybe something is wonky/ not detected properly... switching from dx11 to dx12 might fix it ?
go here http://support.amd.com/en-us/download
put in info....download , install and reboot
then go into control panel -> device or hardware manager and see if its showing the 7400m under Display adapters.
molex .. yuk! heres more like it https://www.newegg.com/Product/Product.aspx?Item=9SIAAXT4V60523&ignorebbr=1&nm_mc=KNC-GoogleMKP-PC&cm_mmc=KNC-GoogleMKP-PC-_-pla-_-Cables+-+Power+Splitter+Cords-_-9SIAAXT4V60523&gclid=EAIaIQobChMI0pWMlMSM1wIVjVYNCh31iw4rEAQYASABEgIuifD_BwE&gclsrc=aw.ds