I haven't been out from Mooloolaba for over a year, but usually the entrance is safe for all vessels. Sometimes is does get a bit silted up. Just have a look at the shape of any waves coming in and you will get a good idea...or just get in line on a good day and follow the convoy
A 4.2 with 40 will be fine out there is good conditions. 10-15 knots or less. For trolling, head out to the Gneerings about 4 km ENE. You will see the 'blinker' marker buoy out there which marks the middle of the inner and outer gneerings. There are a few other reefs in this area - have a look at a chart. There should be plenty of mack tuna at this time of year, maybe some yellowfin and striped tuna.
Marlin can be caught in that area. My first was from Murphies reef in January a few years back. Otherwise keep heading east _as far as you feel safe_
Jeremy