html
<input id="account" value="1234567899876543" />
<input id="account_changed" />
var account = document.getElementById('account');
var changed = document.getElementById('account_changed');
changed.value = new Array(account.value.length-3).join('x') + account.value.substr(account.value.length-4, 4);
output:
1234567899876543
************6543
in MVC3
in place of value put @model.account