lets summarize this then:
There should be no problem entering visa exempt for OP after being out of Thailand for 4 months.
Having a Tourist Visa would improve odds considerably.
A new passport will probably not have any effect (but it could, since the IO would not have to scroll through several pages of stamps and start to get concerned).
To guarantee entry, there are several services available (for a price).
With any questions on arrival it's a good idea to have clear itinerary, accommodation, 20k baht cash etc.
Airline at departure may/will require proof of onward flight.
If denied entry, there is a chance that you could be sent to a neighboring country (if you are guaranteed entry in said country). In other cases, you will be sent home to your home country at your own expense.