There are plenty of FETs out there that meet your needs. You are looking for a power mosfet. So your turn on voltage will be between 2-4V.
Ground is just a reference point, nothing special about it. As long as you can bring the gate voltage up to 4V with respect to the source, the drain to source channel will form and you will have low RDS on. How low? Pick a mosfet that fits your need (look at the datasheet). Here is a hint about that, if the datasheet says RDSON is 0.5 Ohms, that is true only under the conditions that they show they tested it for. If your application does not put it in those exact conditions(unlikely), your RDSon for that part may be higher or lower by some amount.
Here is another hint, you need your "switch" to run 8 amps? Do not choose a fet based on ID continuous current max = 8 AMPS! That device can handle 8 amps but probably for only very low voltages. What you need to do is calculate your power dissipation in the device, pick a fet based on that and make sure it can handle greater than 8 amps. Whatever device you pick, will only survive if it is operating within its SOA curve. This curve includes voltage across the fet, current through the fet and power dissipation.
I have seen to many people fall prey to this. They wire it up, turn it on and it dies even though they never exceeded volts or current rating. Ah, but they exceeded power rating and the die became so hot, it passes the glass transition temperature and the SiO2 melts. If you have this part on a heatsink, it may not even feel hot! There could be a huge temp gradient between die and heat sink.
The message is, do your homework on the operating conditions before you pick a device.
Fairchild semiconductor has nice lines of power fets. You can even get many of them sampled to you for free.
If you are not sure about how to wire the drive up for the device without using "ground", show us your schematic.
Remember, a device doesnt know what "ground" is - only you do and it is inapropriately named to boot! I assure you, my "ground" here is not at the same potential as yours.