What specifically do you mean by "not responding"? (The Activity Monitor is notoriously unhelpful.) What happens if you just wait?
How much RAM? 4 GB may not be enough to run Aperture and a current OS.
How much free space on your system drive? You should leave _at least_ 10% of the drive empty. I recommend 20% for Aperture users.
Have you updated all software to the current versions for your OS? Apple and others are continually fixing bugs and improving performance.
Have you checked your drive for permissions problems or possible early signs of failure? Use OS X's Disk Utility.
Are you having issues with any other programs?
Are you running any uncommon programs or using any uncommon hardware (including tablets, pens, etc.)?
Have you done any of the other steps listed on Apple's Aperture trouble-shooting page?
Programs that hang/crash/don't-let-you-work are frustrating, but most issues can be resolved. Aperture is a resource-intensive application, but when properly maintained will work well.