No go, Vista HCX support max 2025 map tiles. With topo 2008 map, you will hit this hard limit before you reach the 2GB, let alone 4GB.
A deal to die in half an hour
$15.99 - Lowest Ever, Today (april 23) Only A-DATA 4GB MicroSDHC Class 6 with SD Card Adapter
Go to Supermediastore.com, add it to your cart. Use 30% off Special Coupon Code: SUPER30(Exp 04/23/2008) at checkout. There is $5 rebate(expire: 04/23/08). And free shipping.