I would encourage you to get your current electrical problems sorted out first.
Then I would proceed cautiously if trying to add a 4 way flasher. A prior owner had installed an aftermarket 4 way flasher on another classic car I own. It never worked correctly and actually reduced the brightness of the existing turn signals. I will admit that the quality of the wiring connections was marginal at best and there were some ground issues with the turn signals.
I removed it and things got a whole lot better.
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