Weak legs, muscle twitches, memory loss, vision problems, apathy after the birth of 4th child three years ago. Paraneoplastic syndrome?

See your doctor... Your symptoms (regardless of cause) are of concern and you should be seeking care from a physician to discover the cause. Nobody can diagnose you with paraneoplastic syndrome via computer -nor would you want them to as that is a very rare condition that should be managed by a team of physicians (and is secondary to a cancer - which you have not mentioned). Take care of yourself - see your doctor.