''That the external monitor works indicates the major portions of your logic board are good'', however, there are several internal display connectors you could have damaged.
Because you can not see/report on LED #4 no one can be certain if the display connector is damaged (From what you describe I would bet that LED #$ is out). The most likely reason it would be out (and you have the problem you have) is a damaged connector, or cable.
'''This sort of repair is not a DIY project.''' ''(Soldering components on computer logic boards takes special tools, solder and lots & lots of practice) ''An electronics shop might take it on, they would have the tools and experience required, or, the entire logic board could be replaced - this too is an advanced DIY project, even more so than adding an SSD.
''If this answer was helpful'' '''please remember to mark it accepted.'''