Here is one method:
library(dplyr,warn.conflicts = FALSE)
DF <- data.frame(x1 =c (1,0,0,0), x2 = c(0,1,0,0),
x3 = c(1,0,0,1), x4 = c(1,1,0,1))
DF %>% rowwise() %>%
mutate(x5=as.numeric(any(c_across(x1:x4)==1)))
# A tibble: 4 x 5
# Rowwise:
x1 x2 x3 x4 x5
<dbl> <dbl> <dbl> <dbl> <dbl>
1 1 0 1 1 1
2 0 1 0 1 1
3 0 0 0 0 0
4 0 0 1 1 1