You'll get advice and support on 4/32 devices, but the project is effectively no longer providing code changes to support them, or, starting in 2020, no further builds. Not only is the 4 MB flash a problem, but 32 MB of RAM is no longer sufficient for stable operation with current Linux kernels and a minimal set of running applications.
My first suggestion would be, if budget permits, purchase of a decent-quality router with at least 16 MB of flash and at least 128 MB of RAM. They start around US$20, delivered. As I don't own one (yet, I just ordered it), though I have been impressed with their other units, the GL.iNet MT300N-v2 is about 2"x2"x1" and a MTK 7628NN running at 580Mhz. Though I haven't tested the MediaTek (MTK) MIPS-based SoCs (hence why I just ordered one), the performance of a 650 MHz QCA MIPS-based SoC looks like it should be able to handle close to the 100 Mbps limit of the Ethernet phys.
If you've got an earlier version (v1.x) with a 400 MHz SoC, I'd recommend upgrading even more strongly. Performance falls off faster than CPU speed as there are "fixed" things that consume CPU cycles other than just routing/NAT/SQM.
With a v2 version, if you want to stick with it, you absolutely should upgrade to OpenWrt 18.06.05, preferably 19.07 and the ath79 platform (ar71xx is deprecated, hasn't seen a lot of changes since 18.06, as will not exist after v19). There are far too many well-known, severe, actively exploited security vulnerabilities in older kernels, third-party firmware, and even the 802.11 protocols themselves.
With a 4/32 device, LuCI is not only problem for image size, but also RAM consumption. "extroot" can't help with that. It seems to me to be "crazy talk" to run swap on a USB stick with a write speed around 5-10 MB/s.
Keeping a 4/32 device running basically depends on stripping down to absolute minimums and building your own images, either from source or assembling it with pre-compiled packages.