The NexStar mounts have proven to be very popular. Their goto capability, simple operation, light weight and low price (particularly the GT models) make them very attractive as potential mounts for a wide variety of optical tube assemblies (OTAs). How you can go about this depends on which model of NexStar you intend to use. One general thing to keep in mind is how a longer OTA will affect your ability to point at objects high overhead and the weight of the OTA. A little extra weight is not bad if you can successfully balance the OTA.
For the N5/5i/8/8i mount (they are the same), the best option would be "Ray's Brackets". Rather than me trying to explain the bracket, please visit the web site www.buyastrostuff.com. Basically it will allow you to mount any OTA on the N5/5i/8/8i mount.
For the N60/76/80/102/114/130, you could mount another OTA in the existing tube ring. The inside diameter of the N130 tube ring is 150mm, the N114 tube ring is 140mm, the N80 tube ring is 90mm and the N60 tube ring is 63mm. OTAs slightly larger than this would fit just fine. As I get the tube ring measurements of the newer models, I'll post them.
Also for the N60/76/80/102/114/130, there is the Baader Bracket. Baader Planetarium (makers of AstroSolar film - one Sky & Telescopes hot products of 2000) has designed a wonderful bracket available in North America at http://www.alpineastro.com or in Europe directly from Baader - www.baader-planetarium.de. This bracket will allow you to mount nearly any reasonably sized OTA on these scopes. Read my review of the bracket here.
There is currently nothing made specifically for the N4, but the Baader Bracket does physically fit. You will find removal of the existing OTA to be a bit of a challenge. Here is a sketch of the removal process, you will find more details in my book. Basically you must first remove the hand control hook located in the fork arm. Be careful not to loose the screws into the fork arm. After removing the hook, you will see a nut attached to the altitude axis. Remove this nut while supporting the optical tube. At this point, the optical tube, a large gear, and the shaft they both ride on will come free from the fork arm. To remove the gear from the optical tube, you must rotate the gear to align the three holes with the screws found below. The best way to do this is to put the assembly back in the fork arm and push down a bit on the optical tube. Once you can access the screws, remove them and the OTA will finally come off of the bracket attached to the gear. At this point, you are on your own with fashioning a method of attaching an alternate optical tube.
Please note that the diameter of a scope is almost never the same as its aperture, so be sure to check before ordering that OTA of your dreams if you intend to mount it in any specific tube ring.