@bigw you don't need a 4G plan to use 4G/LTE for roaming
In fact, even in Canada, the 3G Plan in fact is just a 3G SPEED plan. All 3G plan customers can still use 4G/LTE network for data except it is in a reduced speed, 3Mpbs. If your phone network type is set to Automatic, likely it is already using 4G/LTE network in Canada
For Us roaming, again, there is no need to upgrade to 4G speed plan to use 4G/LTE roaming. But again, you can better speed if you use 4G speed plan
As to US coverage, it is as good as T-Mobile's coverage (again, no different which plan you use)
Which state and city you are going?
Do you just need data ? or voice as well?
if you need voice, what phone do you have? if your phone is compatible for VoLTE , then you can use VoLTE for voice
