You won't get a supported/ semi-modern version of OpenWrt down to ~3.8 MB flash - nor will it even finish booting successfully (due to a severe lack of RAM).
If you have to ask, it's game over at this point - has been for over half a decade already.
But even if you do know what you're doing and are up to a challenge (combined with quite some experience or a very steep learning curve, with lots of experimenting and debricking/ serial console and soldering iron at hand), it's still hard and way beyond reasonable. I would strongly suggest to check your local second hand markets for cheap offers of supported devices instead, you may be in for a surprise what you can find for 5-15 bucks delivered.