Fix this problem and what is cheapest?rear end air suspension or conversion kit

What is your question?

I have a 1995 Lexus Ls400 and the passenger side rear end air suspension is not working I was looking on ebay for the parts but I do not know if I should buy a conversion kit and replace all four and will that be all I need to fix it or will it be cheaper just to get a new air suspension and will I have to replace all four as well