That will depend on the amplifier size & abilities.
Look on the amp for it's specifications(or research them online). If it is rated as 1ohm stable, I'd suggest running the subs in parallel to achieve a 1ohm load. If it's not, then run them in series, this will present the amp with a 4ohm load.
Just remember that excessive distortion will kill a speaker in fairly short order. To avoid this, the amp must be able to deliver as much or more power than the speakers are rated for.
Example: if you're running them in parallel and the subs are rated at 100W RMS each, be sure your amp is capable of delivering at least 200W RMS at 1ohm.
Ideally, you'll want more than 200W, my basic rule of thumb is the amp should be 40% more powerful than the speaker(s).