A multiple choice test consists of 100 questions. If a student answers a question correctly, he will get 4 marks; if he answers a question wrongly, he will get $-1$ mark. He will get 0 mark for an unanswered question. Determine the number of different total marks of the test. (A total mark can be negative.)
To determine the number of different total marks a student can achieve on a multiple-choice test with 100 questions, where each correct answer gives 4 marks, each wrong answer deducts 1 mark, and unanswered questions give 0 marks, we need to consider all possible combinations of correct, wrong, and unanswered questions.

1. **Define Variables:**
   - Let \( c \) be the number of correct answers.
   - Let \( w \) be the number of wrong answers.
   - Let \( u \) be the number of unanswered questions.
   - We know that \( c + w + u = 100 \).

2. **Total Marks Calculation:**
   - The total marks \( M \) can be expressed as:
     \[
     M = 4c - w
     \]
   - Since \( u \) does not affect the total marks, we can ignore it in the calculation of \( M \).

3. **Range of Total Marks:**
   - The minimum total marks occur when all answers are wrong:
     \[
     M_{\text{min}} = 4 \cdot 0 - 100 = -100
     \]
   - The maximum total marks occur when all answers are correct:
     \[
     M_{\text{max}} = 4 \cdot 100 - 0 = 400
     \]

4. **Possible Total Marks:**
   - We need to determine all possible values of \( M \) between \(-100\) and \(400\).
   - For each \( c \) from 0 to 100, \( w \) can vary from 0 to \( 100 - c \).
   - Therefore, \( M \) can take any value of the form \( 4c - w \) where \( 0 \leq c \leq 100 \) and \( 0 \leq w \leq 100 - c \).

5. **Analysis of Possible Values:**
   - For each \( c \), \( w \) ranges from 0 to \( 100 - c \), so \( M \) ranges from \( 4c - 0 \) to \( 4c - (100 - c) = 5c - 100 \).
   - This means \( M \) can take any integer value from \(-100\) to \( 400 \).

6. **Counting the Total Marks:**
   - The total number of different marks is the number of integers from \(-100\) to \( 400 \), inclusive.
   - This can be calculated as:
     \[
     400 - (-100) + 1 = 501
     \]

The final answer is \(\boxed{501}\).