Am I understanding that you're a current T-Mobile Monthly 4G customer?
If so, you can definitely use the Exhibit II on a M4G pay-as-you-go plan. Your only speedbump may be in pricing. I know T-Mobile stores sell the Exhibit II as a M4G phone for $199.99 with a new activation (and at least $50 pin sale). They don't offer that pricing for existing M4G plans. Instead, you would have to pay the full retail of $329.99.
Now, if you can get the device somewhere cheaper than that, just pop your SIM card in and you're ready to go.