a <- c("I Like What I Know 1959 02e pdfDrama (amazon.com)","My Life 1993 07e pdfDrama (amazon.com)","I Like What I Know 1959 02e pdfDrama more words (amazon.com)")
gsub("(?:(?=\\b\\d{4}\\b)|\\G(?!\\A))\\s*\\K\\b(\\S+)","(\\1)", a, perl=TRUE)
[1] "I Like What I Know (1959) (02e) (pdfDrama) (amazon.com)"
[2] "My Life (1993) (07e) (pdfDrama) (amazon.com)"
[3] "I Like What I Know (1959) (02e) (pdfDrama) (more) (words) (amazon.com)"