you could cron some nightly restart to get around that but really you should use this as a stop gap solution until you get some budget to replace it...
After installing the provided image in this thread I have this output on ssh:
BusyBox v1.33.2 (2022-08-08 06:56:57 UTC) built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt 21.02-SNAPSHOT, r16628-fdb59e128d
-----------------------------------------------------
root@OpenWrt:~# df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 3.0M 3.0M 0 100% /rom
tmpfs 14.1M 232.0K 13.9M 2% /tmp
/dev/mtdblock6 448.0K 236.0K 212.0K 53% /overlay
overlayfs:/overlay 448.0K 236.0K 212.0K 53% /
tmpfs 512.0K 0 512.0K 0% /dev
Looks like some 212K are still free on flash.
So just reading a bit around.
necessary packages seem to be:
- block-mount 22.6 KB
- kmod-fs-ext4 255.1 KB (kmod-fs-exfat only is 36.9 KB in size if that's an alternative)
- e2fsprogs (not available?)
- parted (not available?)
size and availability taken from here: https://downloads.openwrt.org/releases/22.03.5/targets/ramips/rt305x/packages/
As for a newer release a new build is necessary maybe it is possible to somehow integrate support for exroot in a way?
4 MB flash issue
....
...
Why should I build my own image rather than installing packages?Building your own image compresses the packages you need to about half the size compared to installing them after you flash.
Sure. You have all the details in this thread to compile your own image 
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Just for fun I tried to see if I can get the available 22.03.5 image for the wr512-8M (5243214 bytes) shrinked using the firmware-selector
I achieved 4457280 bytes just with most functions still included:
base-files busybox ca-bundle dnsmasq dropbear firewall4 fstools
kmod-gpio-button-hotplug kmod-leds-gpio kmod-nft-offload
kmod-rt2800-soc libc libgcc libustream-wolfssl logd mtd netifd
procd procd-seccomp swconfig uci uclient-fetch urandom-seed
urngd wpad-basic-wolfssl
Still it's to big to fit in 4M. 
I expect LuCi still be to included as the following package list (just extra ones to remove) does result in the exact same image size:
base-files busybox ca-bundle dnsmasq dropbear firewall4 fstools
kmod-gpio-button-hotplug kmod-leds-gpio kmod-nft-offload
kmod-rt2800-soc libc libgcc libustream-wolfssl logd mtd netifd
procd procd-seccomp swconfig uci uclient-fetch urandom-seed
urngd wpad-basic-wolfssl -uhttpd -uhttpd-mod-ubus -libiwinfo-lua
-luci-base -luci-app-firewall -luci-mod-admin-full -luci-theme-bootstrap
Can you share the best config file for 4MB Flash? You can also share separate configs for separate versions ( 21.02, 19.07, 18.6 )
I also have a device 4/32 TL-WR845N-V4