Before signing anything, demand that 4XP clearly define the arbitrage they accuse you of. If you and your group were committing direct violations of well-defined rules in the TOS, 4XP may have the right to cancel profitable trades. Go ahead and get screenshots of the 4XP TOS from their website NOW so they can't change it to ban your trades after the fact.
If all of this was deposited by bank wire, contact your banks and ask them to let you know the best way to place fraud charges from your country. Don't file yet, but get the information handy.
Considering the amounts of cash involved, you may want to consider legal representation.