To calculate the theoretical probability of Lucy picking a 4 from a standard deck of 52 cards, we need to determine the number of favorable outcomes and the total number of possible outcomes.
- Total number of cards in the deck: 52
- Number of favorable outcomes (number of 4s in the deck): There are 4 cards that are 4s (one from each suit: hearts, spades, diamonds, and clubs).
The probability \( P \) of picking a 4 can be calculated using the formula:
\[ P(\text{picking a 4}) = \frac{\text{Number of favorable outcomes}}{\text{Total number of possible outcomes}} = \frac{4}{52} \]
Now, we need to simplify the fraction:
\[ \frac{4}{52} = \frac{1}{13} \]
Thus, the theoretical probability of Lucy picking a 4 from the deck of cards is:
\[ \boxed{\frac{1}{13}} \]