The closest thing you can get to 4G is HSPA+ from Three, to get that you don't need anything special except a device that supports the technology like the new iPad.
Is it 4G? no, T-mobile in the US market it as that, but it isn't proper 4G LTE.
The Three network are upgrading to HSPA+ 42Mbps though, so over the rest of this year you should see speeds really increase, giving you about 10Mbps+ by the end of the year in most areas with HSPA+.
However for 4G LTE you'll have to wait, as the iPad cannot be used for 4G LTE in the UK even when it's released from about 18 months time.
Neither will the iPad work with Orange's LTE that they hope to release later in the year as the iPad doesn't support LTE at 1800Mhz, the only thing that will be supported by Orange at that time is likely to be custom dongles sold by them only.