As per 36.321. For FDD the HARQ RTT Timer is set to 8 subframes. So if tx entity sends data in x subframe then rx entity sends ack/nack in x+4 subframe . then the tx entity can transmit/retransmit in x+8 for same harq process.
As each tx/rx entity need time for processing so to have equal time at both tx/rx entity and harq RTT is 8 subframe ,I think due to this ack/nack comes in x+4 subframe after sending data.
