They arrived at Canterbury in this order: Martin, John, Stephen, Geoffrey.
Consider the problem thusly: since Stephen will tithe 3 pence regardless, between Martin, John, and Geoffrey, 10 pence total was tithed. Martin and John only promise to tithe even numbers of pence, so if Geoffrey beat Stephen, his 3 pence would make the total odd. As the total, ten pence, is even, Geoffrey must not have beaten Stephen. He therefore tithed nothing.
So between Martin and John, ten pence was tithed. The only way to make ten pence, from what we know, is to assume that Martin tithed 6 (by arriving in Canterbury first), and John tithed 4 (by beating Stephen to Canterbury.) Knowing this, we can easily derive their order.