My Viewsonic N3250w tv I have had for 4 years no longer powers up. If you hit the button it lights up around the switch but then almost immediately powers back down. Does this sound like a power supply problem or a switch problem?

Power supply module will be the first thing to check, look for bad caps, find out if it has standby 5vdc and other voltages.
http://s807.photobucket.com/albums/yy352/budm/Viewsonic%20N3250W/
If you are going to DIY and have proper tools, electronics knowledge, and know safety precaution then please read on:

Please see my basic troubleshooting guide here:
http://www.fixya.com/support/r5093881-lcd_flat_panel_tv_troubleshooting_guide

Failed TV and Monitors: http://s807.photobucket.com/home/budm/allalbums

http://en.wikipedia.org/wiki/Capacitor_plague

Parts: www.shopjimmy.com
Capacitors kit: http://lcdalternatives.auctivacommerce.com/Default.aspx he can make you a set of caps.