a 4 PSI switch is fine for this application. This is an Idiot light designed to warn of no pressure, which is the failure point requiring immediate attention. An oil pressure gauge should be used for monitoring running oil pressure and it will identify troubles brewing with your oil pressure. In my experience in automotive (typical oil pressures of 50-60 PSI), you either have normal to low (50-20 PSI) pressure (various causes for the deviation) or no oil pressure from catostrophic failure. I cannot imagine a scenario that would have 5 PSI for more than a couple of seconds on it's way to 0. Further, I believe Lycoming specifies acceptable idle level oil pressure down to 20 PSI. This pressure should concern you as to why, but IMHO, should not flash the disaster light.
In the end, it really depends on your reason for having the light. 30 PSI is quite low for 3500 RPM, but may be perfectly normal for idle. You're not going to find an idiot light to cover this scenario. You need a gauge for this.
Larry