4 days is tight, but you'll get to spend plenty of time at the nearby place to wet your appetite for next time.
Coming from Hervey Bay is a fair ferry ride over. On the inland tracks watch for tree roots sticking out - you have a far greater chance of buggering a tyre than you'll ever look like getting bogged.
I run my unladen hilux's on 20-22 PSI and never in 20 years pf going over there have I looked like getting stuck.
Biggest thing is watch the tides. Also its a fair drive from KF resort to the eastern beach so if you do plan to head up the beach you're best to time leaving the resort so you get to the beach at the right time to suit the tides.
Lake Mackenzie, Central Station, Elli Creek, Maheno Wreck plus the other perched lakes in the area will be about all you'll get to see. If you head too far north don't risk the tides, its a longer drive than you think.
If you go to Lake Mackenzie go early in the morning and have a swim before the bus loads arrive and you'll have the beach mostly to yoursleves.
Have fun, I can guarantee you that once you go over there you'll be hooked for life