Luis is correct it is a MAXIMUM of 3 years. There is no 4 year interval, it is driven by OSHA Hazmat regs. which require training within 90 days of hire, at least every 3 years or when job duties change.
Currently, REFRESHERS are $150 at DEMA or any other time & UPDATES are $35 at DEMA or any other time.
UPDATES are a 1 hour "what is new & what has changed over the last year" review. Taking one every year is the best option to stay in compliance. Should you let the 3 year interval lapse, you need to take the REFRESHER.
The REFRESHER is the full blown course given at a discounted rate in recognition of your PSI rating. If you have any questions contact me directly, send me a priveate message thru SB, or stop by and see me at the PSI booth at DEMA.
Hope this helps!
Gary