Difference between firmwares esp8266_4mb and esp8266 ?

Thanks a lot Mabe for all your replies. I learned a few things, but honestly, I still feel a bit lost between builds, flash maps, etc.
My context, my current project actually run on my esp12-4mb with the basic 512/512 build. I use 8 modules, executed from flash, with FlashString (http://forum.espruino.com/conversations/­290975/) and it's homemade IDE plugin (http://forum.espruino.com/conversations/­313069/). I was trying to figure out if I can run it also on other configurations/builds of esp8266, and how to save and execute code from flash in these.

It seems there is 1700 blocks no graphics in the 4mb combined build from http://www.espruino.com/binaries/espruin­o_1v95_esp8266_4mb_combined_4096.bin.

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Flash map 4MB:1024/1024, manuf 0xc8 chip 0x4016
>process.memory()
={ "free": 1672, "usage": 28, "total": 1700, "history": 0,
  "gc": 0, "gctime": 1.357 }
>var g = Graphics.createArrayBuffer(8,8,8);
Uncaught ReferenceError: "Graphics" is not defined
 at line 1 col 17
var g = Graphics.createArrayBuffer(8,8,8);
                ^
>