EXT4 ON WINDOWS

There are a few tools that can be used on Windows OS's that allow for mounting and reading of data from ext2/3/4 partitions.

Generally speaking, write support is not present, although extfs-windows does, a commercial tool detailed below.
Some tools below may have issues with one feature or more that you use, do yourself a favor and read up on the tools prior to mounting of partitions. For example FS-Driver does not do ext4 or LVM.
Links and descriptions (accurate as of 07 Jul 2017) are provided where the site gives them.

OPEN SOURCE

ext2fsd

Supported Features by Ext3Fsd
flexible inode size: > 128 bytes, up to block size
dir_index: htree directory index
filetype: extra file mode in dentry
large_file: > 4G files supported
sparse_super: super block backup in group descriptor
uninit_bg: fast fsck and group checksum
extent: full support with extending and shrinking.
journal: only support replay for internal journal
flex_bg: first flexible metadata group
symlink and hardlink
* mount-as-user: specifed uid/gid by user

Unsupported Ext3/4 Features
journal: log-based operations, external journal
EA (extended attributes), ACL support
ext2read

Features
Simple UI designed using Qt4/Qt5
View/Read ext2/ext3/ext4 partitions
Linux LVM2 Support
Ext4 Large File support (untested)
Recursively Copy the entire folder or even /
Support for external USB disks
Support for disk and filesystem images For e.g. Wubi users can just open their root.disk file through this program
LRU based Block cache for faster access
* Unicode support

FREEWARE

fs-driver.org

    >   What features are supported?  
    > * Complete reading and writing access to files and directories of volumes with the Ext2 or Ext3 file system.  
    > * Trim support for solid state drives (SSD) on Windows 7/Server 2008 R2 or higher.  
    > * Supports features which are specific to the I/O-system of Windows: Byte Range Locks, Directory Notfication (so the Explorer updates the view of a directory on changes within that directory), Oplocks (so SMB/CIFS clients are able to cache the content of files).  
    > * Allows Windows to run with paging files on Ext2 volumes.  
    > * UTF-8 encoded file names are supported.  
    > * The driver treats files with file names that start with a dot "." character or end with "~" as hidden.  
    > * Supports GPT disks if the Windows version used also does.  
    > * Supports use of the Windows mountvol utility to create or delete drive letters for Ext2 volumes (except on Windows NT 4.0). See also section "Can drive letters also be configured from scripts?".  
    > * Supports large inodes.  
    > * Supports block sizes up to 64KB.  

    >   What features are *not* supported?  
    > * The software is not yet able to access Ext4 volumes.  
    > * Access rights are not maintained. All users can access all the directories and files of an Ext2 volume. If a new file or directory is created, it inherits all the permissions, the GID and the UID from the directory where it has been created. There is one exception to this rule: a file (but not a directory) the driver has created always has cleared "x" permissions, it inherits the "r" and the "w" permissions only. See also section "What limitations arise from not maintaining access rights?".  
    > * Extended attributes are not supported.  
    > * The driver does not allow accessing special files at Ext2 volumes, the access will be always denied. (Special files are sockets, soft links, block devices, character devices and pipes.)  
    > * Alternate 8.3-DOS names are not supported (just because there is no place to store them in an Ext2 file system). This can prevent legacy DOS applications, executed by the NTVDM of Windows, from accessing some files or directories.  
    > * Currently the driver does not implement defragging support. So defragmentation applications will neither show fragmentation information nor defragment any Ext2 volume.  
    > * This software does not achieve booting a Windows operating system from an Ext2 volume.  
    > * LVM volumes are not supported, so it is not possible to access them.

Linux-Reader

COMMERCIAL

extfs-windows

Current price is $19.95 USD for the full version.

As I do not own a physical installation of Windows (all instances are in KVM, and development environments to boot), I cannot say how good of a tool this is firsthand. It exists, and why I am including it here. Although many commercial tools may be better, I prefer open source as I tend to get burnt badly going the closed up route.

Feel free to message me with a review, and whether you wish to have it made public if you do try it, love it or hate it.

New features * 10 days of full speed and functionality usage. The data transfer rate decreases to 5 Mb/s after 10 days, while the product still remains fully functional. In case you only occasionally need access to Linux drives from Windows, the data transfer speed can be increased to the initial rate for up to 25 days with a Facebook post on your timeline or perpetually via in-app purchase.
* The latest Ext4 features support: 64bit, dir_index, extent, extra_isize, ext_attr, flex_bg, has_journal etc. Linux bigalloc, journal_dev, meta_bg features are not supported at the moment

Key features
Full read/write access to any Ext2 / Ext3 / Ext4 partitions. Windows doesn’t support ExtFS partitions at all, but with Paragon’s Ext for Windows, you’ll get full read/write access to any version ExtFS partitions
Easy to install. It takes a few steps only to install the tool and it's ready to use
Easy to use. Just plug in your hard disk with ExtFS partitions to your PC and you can instantly modify files on Linux partition and use your content however you please. It's easier than ever before!
Intuitive user-friendly interface
Run at startup and automount feature
Read/Write support for LVM (Logical Volume Management)
Supports for the latest Windows 10 as well as any Windows since 7
No limitation to maximum file/partition size
Low CPU load during data transfers
Supports languages with non-Roman character