I think in Europe HSDPA, HSUPA and HSPA+ were more consistently called 3.5G, 3.75G and perhaps 3.9G, so 4G and LTE can be synonyms.
In America, operators are marketing HSPA+ as 4G, so then there is a need to specify when 4G is LTE and when not.
1G = NMT
2G = GSM
3G = UMTS
4G = LTE