Can you [play] more than one PlusPower[?] 2, 3, 4?
Well you can have up to 4 in your deck, so if you had all 4 in your hand you could use them all at once yes.
Source: WotC Chat (2000-05-18)

Relevant cards

Note: All applicable cards may not be listed below.
WOTCB1_84
WOTCB2_113
WOTCB2_113
BW1_96
DP1_109
DP3_121
HGSS2_80
PL1_112