I guess I expected it to come up as a Performance since that's what has ARPs 1-5. However it came up as a Song/Pattern instead. A Key On did trigger the ARP for Track 1, but I wasn't able to get the other 3 tracks to sound. What am I missing?

The Motif-Rack XS does not have PERFORMANCE mode. What it has is MULTI mode where the first four PARTS can be placed in a Layer - the parameter is "LAYER 1-4 PARTS"

This parameter: Determines whether or not the Part 1 – 4 voices of the selected Multi are sounded simultaneously as a single Part. If this parameter is set to “on,” the receive channels of the Parts 1 – 4 will automatically be set to the same value as the Receive Ch (Basic Receive Channel) parameter in the Utility Voice MIDI display of the Voice Mode.
Settings: off, on
NOTE: If this parameter is set to “on,” the actual “Receive Ch” settings of the Part 1 – 4 will not be updated from the last settings.

See pages 98 and 40 in your Owner's Manual for more details.

If I'm reading your post correctly, you are trying to move setups you made in the Rack XS to the keyboard XS8 as PERFORMANCES... Unfortunately, the Rack XS does not have a PERFORMANCE mode as the keyboard does, so it cannot format them in this particular way... You can move the MULTIs from the Rack XS to the Motif XS8 as SONG/PATTERN MIXING setups... simply set the MIDI channel of the first four PARTS of the MIXING setup to Channel 1. Then you can play them all together (PLAY them - but not record them...)

On the Motif XS8
Go to the SONG or PATTERN you opened them in
Press [MIXING]
Press [EDIT]
Press Track Select [1] to view PART 1
Press [F1] VOICE
Press [SF1] VOICE
The MIDI Receive channel appears at the top of the screen - it is set to 1
Select PARTs 2, 3 and 4 and also set them to MIDI RECEIVE CHANNEL 1
now you can play all four PARTs simultaneously, when you select TRACK [1]