My guess its not a thermal paste issue i think the problem your temp detector is your problem i don't have antminer s4 but maybe it is the same as antminer s7 or s9 try to follow trouble shooting guide from bitmain here https://support.bitmain.com/hc/en-us/articles/220872007-Possible-problems-for-Antminer-Troubleshooting-for-S7-S9-L3-