更新见OP请求：
只需在grid.arrange()中更改widths，如下所示：

.....
# Arrange the plots
grid.arrange(
  arrangeGrob(a, b, c, d, ncol = 2),
  legend,
  ncol = 3,
  widths = c(8, 2, 0)  # Adjust to desired proportions
)

第一个答案：我们可以使用gridExtra和cowplot包来实现：

library(ggplot2)
library(gridExtra)
library(cowplot)

# Create the 4 plots without a legend
a <- mtcars %>%
  ggplot(aes(x = disp, y = mpg, colour=hp)) +
  geom_point(size=2) +
  geom_smooth(se=F, colour="grey") +
  theme(legend.position = "none")

b <- mtcars %>%
  ggplot(aes(x = disp, y = mpg, colour=hp)) +
  geom_point(size=2) +
  geom_smooth(se=F, colour="grey")  +
  theme(legend.position = "none")

c <- mtcars %>%
  ggplot(aes(x = disp, y = mpg, colour=hp)) +
  geom_point(size=2) +
  geom_smooth(se=F, colour="grey")  +
  theme(legend.position = "none")

d <- mtcars %>%
  ggplot(aes(x = disp, y = mpg, colour=hp)) +
  geom_point(size=2) +
  geom_smooth(se=F, colour="grey")  +
  theme(legend.position = "none")

# Create a separate legend
legend <- get_legend(
  mtcars %>%
    ggplot(aes(x = disp, y = mpg, colour=hp)) +
    geom_point(size=2) +
    geom_smooth(se=F, colour="grey"))

# Arrange the plots
grid.arrange(
  arrangeGrob(a, b, c, d, ncol = 2),
  legend,
  ncol = 3,
  widths = c(4, 4, 1)  # Adjust to desired proportions
)