I assume because Alms Collector doesn't leave the battlefield until state based actions are checked and that won't happen until after Explosion fully resolves.
Yes, Alms Collector will replace Explosion's draw effect, for the reason you mentioned.
The exact order of events is like this:
- Explosion starts resolving.
- Explosion deals 4 damage to Alms Collector.
- Your opponent would draw 4 cards as the result of Explosion's second instruction, but Alms Collector replaces that effect. Instead, you and your opponent each draw 1 card.
- Explosion finishes resolving and goes to the graveyard.
- State-based actions are checked. Alms Collector dies because it has lethal damage marked on it.
- The active player gains priority.
You are correct; you will still get the replacement effect.
State based actions will not be checked until after Explosion fully resolves. So while Explosion is resolving, Alms Collector will have 4 damage, but it (and its static ability) will still be around. When Alms Collector has your opponent draw 4 cards, that effect will be replaced by you and your opponent each drawing a card. Then after Explosion resolves, state based actions are checked and Alms Collector will die.