Can I dehydrate different kinds of herbs on different trays at the same time in my dehydrator? Or, can each tray be a different herb. I am drying sage, oregano, parsley, and thyme. 
Yes, there is no issue with that. If you have specific drying times for each herb then I guess you might need to take them out at different times but I can't see it being an issue. At worst they may absorb a little of each others' odours but the effect should be small and not really change the flavour when used in coming. 