They use the same locks, the same method of mounting, and the same sensors. They're both the same height and width.
The Sonic Shock Plasma is longer by 1.7 inches (4.3 cm), and thus has more area to cling to your equipment. It has twin sirens, making the noise not just lounder, but more disruptive. That said, the '4' model is still noxiously loud.
No, there are many circumstances where the Sonic Shock 4 will fully meet your needs.
There are no hard rules apart from physical space – many LCD projectors have space only to mount a Sonic Shock 4.
Otherwise it's often an economic decision. At current prices most customers choose the Sonic Shock 4 for 37 inch televisions, and mostly the Sonic Shock Plasma for televisions larger than 42 inches.
Other influences? If you've already had a theft, have a more isolated setting or a situation where thieves have time to work, you should consider the 'plasma', even for a smaller television. If you've not yet had a theft, and have a location in well travelled area, the Sonic Shock 4 will be quite adequate, even for a more expensive set.