what is the best android phone under 400$?

If you are looking for an unlocked phone then your best choices under $400 are Xperia SL, LG Nexus 4, Galaxy S II and iphone 4. Iphone 4 is not an android device and Galaxy S II is old and the specs are not as good as Xperia SL and LG Nexus 4. Nexus 4 is way faster and it has newer OS and bigger screen but with nexus 4 you get just 13GB of storage. You can insert a memory card in none of Nexus 4 or Xperia SL so the file transfer becomes a little hard. Xperia SL's internal memory is 32GB. The battery on Nexus 4 also doesn't last for a long time Xperia SL's battery life is not much better either it has just slight advantage over Nexus 4. When performing heavy tasks on Nexus 4 back of the device gets warm. Xperia SL has higher resolution camera. Apart from memory, battery life and camera Nexus 4 is the best but if you care a lot about the memory and battery life then Xperia SL.

http://gsmarena.com/sony_xperia_sl-4894.php

http://gsmarena.com/lg_nexus_4_e960-5048.php

I wouldn't get a Sony phone, they will probably disappoint you(sure as he'll disappointed me). I'm getting the HTC Droid DNA, that's what I would recommend. 443 PPI blows everything else out of the water with a quad core processor, 2 GB RAM, 5 inch super LCD3 screen, and really is just the best phone out there right now. Personally, 5 inches is the perfect sized smartphone for me, but you may want a bigger or smaller one. If you want even bigger, then the galaxy note 2 is the one for you. Great SPen, 5.5 inches, and a ton of features. If you want smaller, then The motorola m isn't bad either, basically a smaller DNA.

DNA costs 200, note 2 costs 300, and I think the motorola m costs 50(not certain though).

i will advise you to purchase nexus 4. This telephone runs on the Android 4.2 JellyBean working device. It has Quad middle this is supported by utilising the aderno 320 portraits chip lets you multitask comfortably and play video games like Iron guy 3 with minimum attempt. It has inbuilt memory of 8 GB alongside with 2 Gb of RAM. It has a 8 MP sensor jointly as the front dealing with one has a a million.3 MP sensor and that's Google telephone So this is usually a benefit and it additionally greater low-priced then different telephones.

I bought Samsung S Duos which is with Android ver 4 and its in USD 300