H10 <- H10 %>%
  rowwise() %>%
  mutate(t10dv = mean(c(fd$DV1_C, fd$DV2_C,
                        fd$DV3_C, fd$DV4_C),
         na.rm = TRUE))

head(H10)

            DV1_C           DV2_C         DV3_C           DV4_C     t10dv
            <int>           <int>           <int>           <int>     <dbl>
1               1               0               0               1 0.3880952
2              -1               0               2              -1 0.3880952
3               0               0               0               0 0.3880952
4               0               2               1               1 0.3880952
5              -1              -1              -1              -2 0.3880952
6              -2               0               0               0 0.3880952