I have done the math for a 4/32 device I owned in February, and the figures didn't look good.
As mentioned there, it is possible to make even harder decisions (and to drop even more), but that was the limit I was going to compromise on, because the system would have become useless (I still wanted it to remain a sensible router/ AP, even if all the convenience (luci) was already gone) to me beyond that point (and the efforts required already exceeded its remaining value). In your case, you'd even have to do the initial porting to ath79, which -while certainly possible- will add yet another layer of complexity. Keep in mind that 32 MB RAM is as much of a problem here, as the limited flash size.
If you still want to take it head on, I would suggest to replace the 4 MB flash chip with a compatible 8 MB model first, as this would make your task easier - while doing so would complicate getting the support merged into OpenWrt, as OpenWrt doesn't take patches for one-off, user-modified devices, that question is out of the question for 4/32 devices at this point anyways. But the ~1-2 bucks for the new flash chip and the 'half an hour' worth of soldering would make your endeavour much easier (and you will need serial console access for this kind of porting anyways).
Don't get me wrong, I won't recommend the above - but it's at least a more reasonable way forward that trying to squeeze modern OpenWrt into 4 MB flash (but yes, 32 MB RAM will also become a serious issue once you start up the wireless interfaces, maybe even before that).
From an economic point of view you could get much better (plenty of flash & RAM, 802.11ac/ ax wireless) devices both on your local used markets or even new from a seller for very little money (decent used gear starting in the 5-15 EUR range if you're lucky - brandnew 802.11ax stuff starting around 15 EUR (mt7621a+mt7915DBDC, e.g. dap-x1860, covr-x1860, wsm20) to ~30 EUR (filogic 820, e.g. ax3000t). That is considerably cheaper than buying:
- soldering iron (e.g. Miniware TS101, starting around 40 EUR in a sale)
- solder, flux, solder wick, 2.54mm DuPont pins (~10 EUR)
- 3.3V serial console (e.g. cp2102, ch340g, fake ft232l, ~1.50 EUR)
- 3.3V(!) spi-nor programmer (e.g. ch341), soic-8 clamp (~6-10 EUR)
- 2-4 EUR for the new 8 MB spi-nor chip itself (don't go higher than this, you only get into trouble with the 32 MB RAM while flashing and may have to fight against bootloader/ PCB issues)
From a practical point of view, >=802.11ac brings a massive improvement over 802.11n device, you get
- 1 GBit/s ethernet
- 2.4+5 GHz concurrent dual-band support
- 200-350 MBit/s wireless throughput (over 5 GHz)
- and 802.11ax pushes this to 600-750 MBit/s over the air
- more flash/ RAM
- more CPU performance
so even without the flash/ RAM constraints, replacing 802.11n devices with something newer really does make sense on merits alone. Keep in mind that the airwaves are a shared medium, so your 802.11n keeps the channel (actually three channels at once) busy, while transmitting at slow speeds, preventing all faster gear in the vicinity to transfer this amount of traffic in much shorter time.