FWIW, I got the first Shingrix shot 4 years ago, but neglected to get the second. So, I asked Perplexity:
”Yes, you can still get the second dose of Shingrix even if it has been 4 years since your first dose. The CDC recommends the second dose be given 2 to 6 months after the first, but if more than 6 months have passed, you do not need to restart the series; just get the second dose as soon as possible. Studies show that the vaccine remains effective even if the second dose is delayed beyond 6 months, and completing the two-dose series provides much better protection than only one dose”