I posted my build on the official forums because people were saying they couldn't add any points to their 4th job, but here it is:
Job 1 10 Dark Sight (need to max to unlock Advanced Dark Sight in 3rd job) 5 Flash Jump 10 Side Step 4 Bandit Slash
Job 1+ 10 Katara Mastery (need to max to unlock Dual Wield Expert in 4th job) 10 Self Haste 15 Tornado Spin
Job 2 10 Fatal Blow 20 Slash Storm (need to max to unlock Bloody Storm in 3rd job) 20 Channel Karma 5 Physical Training 10 Katara Booster
Job 2+ 5 Flash Bang 8 Upper Stab 20 Venom (need to max to unlock Toxic Venom in 4th job) 20 Flying Assaulter
Job 3 20 Bloody Storm 5 Advanced Dark Sight 20 Mirror Image (need to max to unlock Mirrored Target in 4th job) 20 Chains of Hell 5 Life Drain 10 Enveloping Darkness 20 Shadow Meld
Be sure to pay attention to the pop up where it says which jobs you have to put SP into. It's really useful. I personally made this build and at level 140, have been able to put SP into my 4th job skills.
And the skills are not maxed in order. You have to go back and SP into skills from earlier jobs sometimes. It's confusing, but if your build looks like this by the time you get to 4th job, you'll do fine.
Hmm, I don't know if Dual Bladers get 4th job at level 100 like everyone else since their job system is different, but I'm guessing you're 20 levels into 3rd job? I'd say max Mirror Image first, then Bloody Storm, then Shadow Meld and at least 1 into Chains of Hell if you plan to fight any types of bosses such as in PQs, then work on Enveloping Darkness, Life Drain, Advanced Dark Sight then finish of by maxing Chains of Hell.
It's been quite a while since I was third job so I'm not exactly sure how I did my build as I leveled up, but what I just said sounds good.