On the fourth day of my zpak my bronchitis isnt improving! What can I do?

Another antibiotic. If you have not improved after 4 days on a z-pak.. Then you 1) need to see your doctor again and 2) likely will need another/different antibiotic. You can have residual reactive airway disease after bronchitis so it may be that as well.. But if you don't think you are better... See your doctor again.
Re-evaluation. Your bronchitis might also be something else than bronchitis - post nasal drip, for example - or might be viral. Get re-evaluated!
Re-Eval by doctor... There may not be an infection at all. If you have minimal to no fever, you may have viral acute bronchitis, which will resolve on its own over the next few days. You may also have allergic bronchospasm causing these symptoms and treatment would include bronchodilator and steroid inhalers. You may have post-nasal drip causing bronchospasm and would need a nasal spray added to the inhalers.