No. Gionee A1 Plus video recording is restricted to 1080p resolution. You can record Full HD videos but you cannot record 4k videos.
Yes, Gionee A1 Plus does support OTG.
Yes, Gionee A1 Plus has stereo speaker. One speaker is located on the bottom of the phone and ear-piece also works as speaker and thus you will get excellent stereo sound output.
Yes, Gionee A1 Plus is made of Aluminium Alloy.