Difference between firmwares esp8266_4mb and esp8266 ?

Thanks again Mabe, for your reply. I made a copy/paste mistake earlier, now it's updated.

The esp8266 4mb not combined version (found here) get me :

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| http://espruino.com
 1v95.20 Copyright 2017 G.Williams
Espruino is Open Source. Our work is supported
only by sales of official boards and donations:
http://espruino.com/Donate
Flash map 4MB:512/512, manuf 0xef chip 0x4016
>

It says 512/512. Is it the first or the second build ?

Using the 4mb combined, from the WebIDE, I managed to upload 16kb of code, 4kb by 4kb, and the total took 1300 blocks, and it can't be saved with save(), too big :

> _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| http://espruino.com
 1v95.20 Copyright 2017 G.Williams
Espruino is Open Source. Our work is supported
only by sales of official boards and donations:
http://espruino.com/Donate
Flash map 4MB:1024/1024, manuf 0xc8 chip 0x4016
> process.memory()
={ "free": 360, "usage": 1340, "total": 1700, "history": 358,
  "gc": 0, "gctime": 1.351 }
>save()
=undefined
Erasing Flash.....
Writing..................
ERROR: Too big to save to flash (15900 vs 12284 bytes)
Deleting command history and trying again...
Erasing Flash.....
Writing..................

I fail to grasp the rationale behind these 16 pages for saved code and how are we supposed to fill them. Ok, I can fill 12kb of the 64kb reserved for saving code. Is there a way to use the resting 52kb ? is there a way to execute them from flash (like with save-on-send/E.memoryArea) ?