Replied questions:
1. S4 is compatible with p2pool?
Yes
2. When do we release the latest firmware?
Engineers have released one version of firmware merged cgminer 4.6.1 today, which is now being tested. And we will upload it to bitmaintech.com when it is tested running stably.
Please PM us if you have any request about current verson firmware.
Updated cgminer to v4.6.1 for S4
https://www.bitmaintech.com/support.htm?pid=00720140930114518599JXGHWWD80660
3. how to upgrade latest firmware to S2?
Now the latest firmware for S3 and S4 is ready to be released soon, that one for S2 is being developed. Stay tuned here.
1. S4 is compatible with p2pool?
Yes
P2pool works. 
Pools were selected from http://p2pool-nodes.info/
If there is a specific pool, you want a try, please let me know!
https://bitcointalk.org/index.php?topic=796839.msg9075999#msg9075999 Pools were selected from http://p2pool-nodes.info/If there is a specific pool, you want a try, please let me know!
2. When do we release the latest firmware?
Engineers have released one version of firmware merged cgminer 4.6.1 today, which is now being tested. And we will upload it to bitmaintech.com when it is tested running stably.
Please PM us if you have any request about current verson firmware.
Updated cgminer to v4.6.1 for S4
https://www.bitmaintech.com/support.htm?pid=00720140930114518599JXGHWWD80660
3. how to upgrade latest firmware to S2?
Now the latest firmware for S3 and S4 is ready to be released soon, that one for S2 is being developed. Stay tuned here.