Difference between firmwares esp8266_4mb and esp8266 ?

Thanks @MaBe for your reply, Things are ... more confusing now :P
Do I understand correctly, there is 3 different builds for esp8266 ?

1 / build 8266
found here : http://www.espruino.com/binaries/espruino_1v95_esp8266/. I guess it's the one decribed by this file : https://github.com/espruino/Espruino/blob/master/boards/ESP8266_BOARD.py. Has a 1700 vars and no Graphics. Available free flash total is 3mb+40kb, the 40kb are under 1mb usable to run modules from flash with E.memoryArea(). This is the build I usually use, with no problem. I install it with :

python esptool.py --port COM20 --baud 115200 erase_flash
python esptool.py --port COM20 --baud 115200 write_flash --flash_freq 40m --flash_mode dio --flash_size 32m 0x0000 boot_v1.6.bin 0x1000 espruino_esp8266_user1.bin 0x3FC000 esp_init_data_default.bin 0x3FE000 blank.bin

and get

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| 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
>process.memory()
={ "free": 1677, "usage": 23, "total": 1700, "history": 0,
  "gc": 0, "gctime": 1.352 }
>require("Flash").getFree()
=[
  { "addr": 487424, "length": 4096 },
  { "addr": 524288, "length": 4096 },
  { "addr": 1011712, "length": 36864 },
  { "addr": 1048576, "length": 3129344 }
 ]

2 / build 8266 4mb
found here : http://www.espruino.com/binaries/espruino_1v95_esp8266_4mb/. I guess it's the one decribed by this file : https://github.com/espruino/Espruino/blob/master/boards/ESP8266_4MB.py. Has a 1600 vars and Graphics.
I install it with the exact same command line, maybe it is not correct ? I get a very similar result, except that when I store something in the 6 first flash pages, it get erased on save(). Any idea why ?

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| 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
>process.memory()
={ "free": 1572, "usage": 28, "total": 1600, "history": 0,
  "gc": 0, "gctime": 1.726 }
>require("Flash").getFree()
=[
  { "addr": 487424, "length": 4096 },
  { "addr": 524288, "length": 4096 },
  { "addr": 1011712, "length": 36864 },
  { "addr": 1048576, "length": 3129344 }
 ]
>Graphics
=function () { [native code] }

3 / build 8266 4mb combined found here : http://www.espruino.com/binaries/espruino_1v95_esp8266_4mb_combined_4096.bin. I think it's the one you are referring to. is it described somewhere ? It has 1700 vars, no Graphics. I used the "4mb-c1" you posted in the other thread, and it worked perfectly. I understand the flash available is only the last 2MB, so not possible to have modules running from flash.

python esptool.py --port COM20 --baud 115200 write_flash --flash_freq 80m  --flash_mode dio --flash_size 4MB-c1  0x0000 boot_v1.6.bin 0x1000 espruino_esp8266_user1.bin  0x3FC000  esp_init_data_default.bin 0x3FE000 blank.bin

(updated following)

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| 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": 1672, "usage": 28, "total": 1700, "history": 0,
  "gc": 0, "gctime": 1.358 }
>require("Flash").getFree()
=[
  { "addr": 2097152, "length": 1048576 },
  { "addr": 3145728, "length": 262144 },
  { "addr": 3407872, "length": 262144 },
  { "addr": 3670016, "length": 262144 },
  { "addr": 3932160, "length": 262144 }
 ]

For this version, you mention 16 pages of saved code available, that's 64kb. How is it possible to store as much code in them ? If I use the WebIDE, the board throw a UNFINISHED STRING when the transfer reach 10kb. I guess there is no memory for larger transfer. Is there another way ?

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| 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
>Uncaught SyntaxError: Got UNFINISHED STRING expected EOF
 at line 1 col 15
E.setBootCode("Modules.removeAllCached(),Modules.addCached('...
              ^
New interpreter error: LOW_MEMORY,MEMORY
>