It isn't not clear how much you already know, we don't want to reprint the entire manual here... so we'll give you an overview and then, if you are unsure or unfamiliar, let us know, we can fill in any details with more specific routines.

What you'll want to be able to do is create the 4-bar phrase in the MOXF Sequencer using the Voice you want the phrase play. Then we will convert that recording to a User Arpeggio phrase and assign it to that Voice as a Part in a Performance. This way you can trigger it whenever you need it by touching a specific key/velocity to trigger playback. You can set ARP phrases to play "fixed note" (means it plays exactly as you played it), and you can set them to play once, when triggered... rather always looping and chord adjusting as most Arps do.

Go to [PATTERN] Mode
Setup and record your 4 measure phrase to Track 1

When you have the phrase recorded exactly as you like, you can create a User Arp.
Press [JOB]
Press [F5] TRACK
Cursor down to find the Job06: Put Track to Arpeggio

You'll need to setup the dialog box that appears as follows:
-------------------------------------------------------------------------------
06: Put Track to Arp ____ SecA Meas001-004
001 [---put a name here---]
MainCtgry _ _ _ ARP Track 1
SubCtgry _ _ _ Track = TR01
OrgNotesRoot _ TYPE = Fixed
-------------------------------------------------------------------------------

This creates a User Phrase, in USER 001, which can be used to "play" your phrase whenever you trigger it. You can define the trigger using the ARP Note Limit and Velocity Limit parameters. You will want to set the ARP so that it behaves as you require.

NEXT: You will need to create a Performance, as you know a Performance can have four Voices, one in each of its four Parts.
Select the sound or sounds you want to play and make sure one of the montage is the Voice that you want to play the ARP phrase when you trigger it.

For example, if this ARP Part is in Part 4 you can assign an ARP phrase to it as follows:

In your Performance, on the main screen
Press the dedicated [F5] ARP ED
This will take you to Arpeggio Edit
Press [F1] COMMON
Set your Tempo
Set the ARP SWITCH = ON

Press [F2] TYPE
Set the BANK = USER
TYPE = 001

Press [F3] MAIN
Use buttons [1]-[4] to PART SELECT press [4] or wherever your ARP Phrase is to be assigned
You will see the PART number in the upper left corner of the screen
Set the Part SWITCH = ON
Set the HOLD = ON (this means you will not have to keep your finger of the note, the Arp will "hold" the note for you)

Press [F4] LIMIT
Here you can select which key or keys you want to use to trigger the phrase. You might opt for the lowest key, the highest key, or you might opt to put it anywhere you like but only at a velocity of 127... try different things.
C-2 is the lowest possible MIDI note and G8 is the highest possible MIDI note... If you have a MOXF6 then you have C1-C6 as your key range. If you have a MOXF8 you have A-1 through C7 as your keyboard range.
Move the cursor to highlight a NOTE LIMIT
Hold [SHIFT] and touch the KEY you want enter
The ARP NOTE LIMITS let you decide which key or keys will TRIGGER the Arp phrase.

Press [F5] PLAY FX
There are several screen here notice the word "PAGE" in the middle top... this indicates that you can use the LEFT or RIGHT CURSOR to see additional screens. On the second PAGE (to the right)
Set LOOP = OFF
This will make the phrase play as a one shot

You can then setup the rest of the PERFORMANCE sounds to either play or not play on your "hidden" trigger not, but setting the PART NOTE LIMITS for each say you set the ARP phrase to play anytime you hit the lowest key on your keyboard. You could then program the other PARTS to NOTE sound when this key is pressed (as an example).

The PART NOTE LIMITS are different from the ARP NOTE LIMITS.
To find the PART Note Limit
While in EDIT
Press the numbered button [1]-[4] to select the PART
Press [F1] VOICE
Press [SF3] LIMIT
This PART Note Limit controls what keys will make sound for each of the PARTS.

When you have the Performance setup as you require, Name it and [STORE] it to your instrument's internal User memory.

Hope that helps. If you get stuck, post back here.