Not a given
Indeed, it's not a given that 4G is faster than 3G. There are markets here in the US where a companies 4G LTE network has saturated to the point the 3G is definitely faster. Why they don't shift load back and forth, that I do not know. This is even true with the CDMA carriers -- where a channel of EVDO 3G maxes out at 3.1mbps... you can find some spots where 4G will get like 1mbps but the 3G get 1.5-2mbps.
The reason the 4G is not too exciting right now? Amounts of spectrum deployed. LTE with 2x2 MIMO gets 37.5mbps peak in 5x5 (5mhz down, 5mhz up) versus HSPA+'s 21mbps. That's almost double the capacity. But, right now there is probably quite a bit more spectrum running HSPA+ than LTE.
As the article mentions, current phones also don't support carrier aggregation; however, I think the improvements from carrier aggregation may be a bit overblown. Here in the US, you have areas where LTE is pretty saturated, and people thinking carrier aggregation will double their speeds. I think they won't. CA will double your peak speed (if you get 2 channels the same width as the 1 you get now). CA will increase speeds to a lesser extent on a realistic network. On a heavily loaded network, your device will be limited as to how many resources it can use whether it gets those resources from several LTE channels or all from one channel, and I don't really think CA will help at all.