How to save to png multiple plots created at one R script - r

Question

I have few separate plots in one R script that I'd like to save to png files. I run the script at the terminal and out of the 4 plots, only the last one was created as png file.
How to fix it so it will create and save all 4 plots as png files?
library(data.table)
library("ggplot2")
setwd("C:\\Users\\Desktop\\R\\figure_1A-D")
final_df_older <- fread("C:\\Users\\Desktop\\R\\older.csv"),
select=c(3,4))
png("Figure 1a - older_samples.png", width = 500, height = 400)
figure_1_wo_new <- ggplot(data = final_df_older, aes(x=final_df_older$`quntity`, y=final_df_older$`count`))
dev.off()
list.files(pattern = "png")
getwd()
final_df_all <- fread("C:\\Users\\Desktop\\R\\all.csv"),
select=c(3,4))
png("Figure 1b - all)samples.png", width = 500, height = 400)
figure_1_all <- ggplot(data = final_df_all, aes(x=final_df_all$`quntity`, y=final_df_all$`count`))
dev.off()
list.files(pattern = "png")
getwd()
final_df_new <- fread("C:\\Users\\Desktop\\R\\new.csv"),
select=c(3,4))
png("Figure 1c - new_samples.png", width = 500, height = 400)
figure_1_new_only <- ggplot(data = final_df_new, aes(x=final_df_new$`quntity`, y=final_df_new$`count`))
dev.off()
list.files(pattern = "png")
getwd()
library("gridExtra")
library("grid")
png("Figure 1d - merged_plot.png", width = 1500, height = 500)
merged_plot <- grid.arrange(figure_1_wo_new, figure_1_all,figure_1_new_only, ncol=3,
top = textGrob("Merged plot",
gp=gpar(fontsize=30,font=3,cex=1,col="darksalmon"),vjust=0.4))
dev.off()
list.files(pattern = "png")
getwd()