I just opened a pull request for a reshaped implementation of the ANT build script to run test in parallel on the machine executing the tests. So you can run two Chromes at the same time and speed up the test by factor 2. If you have more resources locally, more browsers are possible too.
Each browser eats at least one local physical cpu core. So even with 2 more hyper-threaded core, you won’t gain anything on a 2+2 machine.