What is the difference between Bluetooth 2.0, 3.0 and 4.0 ?
In general, each revision of Bluetooth improves upon power consumption, data rate, security, pairing reliability, ease and speed, etc. If given an option, always choose the device with the highest Bluetooth version. In addition to the versions, there are some specific optional features and differences explained in detail below:
Bluetooth 1.x - Basic rate bluetooth
Bluetooth 1.x has theoretical max data rate of 1Mbps. Those are relic devices that are nearly extinct, useful as collectors items :)
Bluetooth 2.x - Basic rate + EDR (optional)
The most popular variant, especially 2.1. Bluetooth 2.x introduces the optional Enhanced Data Rate (EDR) capability. Devices with basic rate + EDR (optional) have theoretical maximum data rate of 3 Mbps. The 2.1 version makes it easier to pair with devices from different manufacturers and increases the reliability of the pairing process. 2.1 devices feature Secure Simple Pairing (SSP): this not only improves the pairing experience, but increases the use and strength of security. SSP is essential for protecting data against man in the middle (MITM) attacks.
Bluetooth 3.x - Basic rate + EDR (optional) + HS (optional)
Introduces support for an alternate lower layer, i.e. all applications that are available with Bluetooth radio can also be run over an alternate radio (802.11). This optional feature is called High Speed (HS), and, as the name suggests intended to improve speed, with theoretical HS max data rate of 24Mbps when using 802.11 WiFi physical layer. 3.0+HS devices are not very common.
Bluetooth 4.x - Basic rate + EDR (optional) + HS (optional) + LE (optional)
Introduces support for collecting data from very low-rate devices. The main intent of this feature, called Low Energy (LE), is to aggregate data from various sensors, like heart rate monitors, thermometers etc. The commercial name of this Low Energy (LE) feature is Bluetooth Smart.
Note that most of the features introduced with newer revisions are optional, and one should pay attention to the added feature, not only the Bluetooth version. For example, Bluetooth 3.0 does not necessarily have the optional HS capability, it should be advertised as 3.0+HS. Also, a Bluetooth 3.0 or 4.0 device may not even support EDR, or SSP (Secure Simple Pairing introduced in v.2.1), as they are optional features.