TDM440E or TDM404E

Good day to all !!
Im just new with telephony, VoIP and freepbx so please bear with me…
I have trixbox running now and its doing fine. I uses xlite softphones and some nokia E65 as hardphones.
I want to have outgoing and imcoming calls on this locals lines.
Our company wants to have four concurrent incoming or outgoing calls.

We are going to apply four lines from the telco here for this set-up. So what im going to use so the lines from the telco can be integrate to our trixbox ? Is it TDM440E which has 4 FXS , 0 FXO or TDM404E which has 0 FXS , 4 FXO .

For now im really confused with the differences of FXS and FXO.

Any thoughts are very much welcome.

Thanks and more power!!

Best regards,

what is FXO and what is FXS: