boxplot(c(4.738094, 21.736701, 6.716363, 10.306583, 4.757640, 6.265024), plot = FALSE)$stats
## [,1]
## [1,] 4.738094 <<== It's definitely there but the lower bound of the IQR is almost the same as min val
## [2,] 4.757640
## [3,] 6.490694
## [4,] 10.306583 <<== Upper bound of IQR == max val
## [5,] 10.306583
Если вы увеличите окно графика, размер сетки будет достаточен, чтобы увидеть нижний IQR:
boxplot(
c(4.738094, 21.736701, 6.716363, 10.306583, 4.757640, 6.265024),
horizontal = TRUE
)