Flip cover:

Most of the accessories in the official site of Samsung is centred around protecting the smartphone. Of course, its not a cheap devoice. It will create a big hole in your pocket. Though the device has Gorilla Glass protection and plastic rear, nobody will want to risk such an expensive smartphone. And the consumers will be buying it for a long time. So, naturally, they will want to keep the device as new for a long time. So, Samsung will be selling a Flip Cover. It will provide protection for both the front and the rear simultaneously. The Flip cover will be available in seven colour variations- Black Mist, White Frost, Light Blue, Pink, Green, Orange and Yellow.
S View Flip Cover:

It is a variant of the above mentioned flip cover. This S View flip cover is meant for those who wanna have a peek at the screen of the smartphone without having to flip up the front cover that protects the screen. This flip cover will be having a clear window at the top of the front cover, which will give a glance to check the date, time, battery status, text messages, and incoming call details without having to lift the cover. When the cover is lifted, an auto wake up system will turn the handset on. So, there is no need to press any button to check all these things, unless you have set any passwords. There will be only two colour variations to this S Vie Flip Cover- Black Mist and White Frost.
Protective Cover+:

This can be called a more budget friendly cover. This cover will be protecting the rear side and both the sides of the smartphone, leaving the front screen fully exposed. This will give more grip and thickness to the handset. This will be available in five different colours- White, Navy, Green, Blue and Pink.
Spare Battery Charging system:

This is something which all other smartphones lack. Samsung has announced a Spare Battery Charging System for its Galaxy S4. This will provide a chance to charge a second battery of 2,600 mAh. This system can charge the battery without the handset. That means, you can charge the battery out side the hand set. This will give you a chance to changeover the battery more quickly.
There are some other accessories too, which are mentioned in the official site of Samsung but not yet confirmed. They are:
Classic Pouch:

This is the most commonly used pouch design. You can slide in the phone when it is not in use in to this pouch, which will protect it from finger prints, dust etc.
Wireless charging pad/cover:

The Samsung Galaxy S4 doesn't have built-in wireless charging. That is why Samsung is shipping this accessory separately. The charging pad is super slim as per the claims of the company.
Bluetooth headset:
To enhance the wire less, hands free facility, Samsung is releasing Bluetooth enabled headset for Galaxy S4. But, unlike other headsets, this is not a subtle one. It is a bit longer but thin.
Headset:

Samsung will be releasing wired headset for Galaxy S4, which will be an iPod-like set of earphones, with full volume and track-skipping/call answer/call end button.
HDTV Adaptor:
As Samsung Galaxy S4 comes with excellent TV sharing capabilities, the company has introduced MHL HDTV adaptor. All that you will have to do is to simply hooking the Galaxy S4 with your TV.
S Band:

The Samsung Galaxy S4 packs in a lot of health and fitness features inside its S Health app. The S Band is for your wrist and it can be used to measure activity as well as sleep efficiency. The data will be then transferred to your Galaxy S4 via Bluetooth. The S-Band is also waterproof.
Body Scale:

It will connect via Bluetooth to transmit your weight to the S Health app on your Samsung Galaxy S4.
HRM:

The Heart Rate Monitor is yet another accessory releasing with Galaxy S4. It works with the Running Pro application and connects wirelessly to your Samsung Galaxy S4 to transmit data from your workout.
These are the accessories expected to be released along with Samsung Galaxy S4.