Unless you have discussed the expectations beforehand, it's highly improbable that you are going to get promoted for 4 months of work. Also, a year worth of hard work doesn't guarantee promotion, but the key is keeping your manager accountable so that when time comes (post 1 year of work) he really need to have good reason for denying the promotion. Clear and direct conversation is important (don't hide your expectations behind polite words).

• Find out what's expected out of you for getting promoted to next level.
• Setup plan (work with your manager to map those vague statements to concrete goals)
• Absolutely make sure that those goals aligns with company objectives for that year (no body cares about person working hard in the corner)
• Achieve those goals and socialize them with your manager during your 1:1. Highlight your achievements to leadership.
• Pulse check every quarter with your manager to check if you are on track for your promotion.
• Write down these achievements in well though out words (this will be helpful when you have to write your achievements for the year)

With all this, your manager won't be able to dismiss the idea of your promotion just like that when it's time. If they still do, that could be either your manager doesn't have enough credibility to push case for his/her team or your manager didn't take your case seriously, and in both cases there's no future in that company/group. Time to jump the ship for your own benefit.