If you're aiming to go to SG big 4.. Start applying early. That will give you time to apply 2-3 rounds before your planned move date and will give you time to look for other opportunities. Would be good if you have contacts within that can refer you in as well..

It's never easy for tax professionals to move to the big 4 unless you have good achievements in your resume imho

It's a trade-off. There's a high chance that the SG big4 will offer you a rank that's 1-2 lower hence it's better to move early before you start earning too much and feel the pain when your pay drops down again. But on the other hand a better resume, better work experience will lead to higher chances at getting a job in SG. That's why I recommend applying as early as possible.

Sadly it's a bad year for job-hunting though (what with the slow economy and all) but it's a good time to brush up on your resume and practice your interview skills in preparation for when the economy picks up.