you are asking to Sum rather than to Merge ...
cols_merge, wont sum for you, it will stick character strings together , its in the gt package.
unite from tidyr similarly wont sum for you.
a base R way assuming you want to find the row sum of all your columns is rowSums which is in base R.
(example_df <- structure(list(V1 = c(3L, 1L, 8L, 7L, 2L),
V2 = c(8L, 3L, 6L, 9L, 5L),
V3 = c(4L, 6L, 6L, 6L, 9L),
V4 = c( 10L, 6L, 4L, 2L, 9L)),
class = "data.frame", row.names = c(NA, -5L)))
example_df$result_of_sum <- rowSums(example_df)
example_df
However, I would use tidyverse package (or just dplyr) which generally makes my work easier.
library(dplyr)
example_df %>% mutate(
tidy_result = rowSums(select( .,
starts_with("V")
)))