The cards are fully compatible with 4G / 3G and 2G, so using one in a 4G phone is not a problem.
You can't add a SIM card to your existing Direct Debit as Virgin Mobile appears to operate separately when it comes to such things. You will need another account and Direct Debit following a credit check.
You could go for one of the Oomph Broadband tariffs that include a free SIM but again it will be on a separate account.
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