Difference between firmwares esp8266_4mb and esp8266 ?

The main reason is that with a 4MB board it is possible to use a 4MB:1024/1024 flash map.

With this map type a max flash size of 962560 byte and 16 pages to save code is defined.

This flash areas are free to use:

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|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| http://espruino.com
 1v95.3469 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 0xe0 chip 0x4016
>require("ESP8266").getFreeFlash()
=[
  { "addr": 2097152, "length": 1048576 },
  { "addr": 3145728, "length": 262144 },
  { "addr": 3407872, "length": 262144 },
  { "addr": 3670016, "length": 262144 },
  { "addr": 3932160, "length": 262144 }
 ]