Let $f(x)=|\log(x+1)|$ and let $a,b$ be two real numbers ($a<b$) satisfying the equations $f(a)=f\left(-\frac{b+1}{a+1}\right)$ and $f\left(10a+6b+21\right)=4\log 2$. Find $a,b$.
1. **Understanding the function and domain:**
   The function given is \( f(x) = |\log(x+1)| \). The domain of \( \log(x+1) \) is \( x > -1 \). Therefore, \( a \) and \( b \) must satisfy \( a > -1 \) and \( b > -1 \).

2. **Analyzing the first condition:**
   The first condition is \( f(a) = f\left(-\frac{b+1}{a+1}\right) \). This implies:
   \[
   |\log(a+1)| = \left| \log\left(-\frac{b+1}{a+1} + 1\right) \right|
   \]
   Simplifying inside the logarithm:
   \[
   -\frac{b+1}{a+1} + 1 = \frac{-(b+1) + (a+1)}{a+1} = \frac{a - b}{a+1}
   \]
   Therefore, the equation becomes:
   \[
   |\log(a+1)| = \left| \log\left(\frac{a - b}{a+1}\right) \right|
   \]

3. **Analyzing the second condition:**
   The second condition is \( f(10a + 6b + 21) = 4 \log 2 \). This implies:
   \[
   |\log(10a + 6b + 21 + 1)| = 4 \log 2
   \]
   Simplifying inside the logarithm:
   \[
   |\log(10a + 6b + 22)| = 4 \log 2
   \]
   Since \( 4 \log 2 = \log(2^4) = \log 16 \), we have:
   \[
   |\log(10a + 6b + 22)| = \log 16
   \]
   This implies:
   \[
   10a + 6b + 22 = 16 \quad \text{or} \quad 10a + 6b + 22 = \frac{1}{16}
   \]
   Solving the first equation:
   \[
   10a + 6b + 22 = 16 \implies 10a + 6b = -6 \implies 5a + 3b = -3
   \]
   Solving the second equation:
   \[
   10a + 6b + 22 = \frac{1}{16} \implies 10a + 6b = \frac{1}{16} - 22 \implies 10a + 6b = -\frac{351}{16}
   \]
   The second equation is not practical since \( a \) and \( b \) are real numbers and the left side is an integer.

4. **Solving the first equation:**
   \[
   5a + 3b = -3
   \]

5. **Combining both conditions:**
   From the first condition, we have:
   \[
   |\log(a+1)| = \left| \log\left(\frac{a - b}{a+1}\right) \right|
   \]
   This implies two cases:
   \[
   \log(a+1) = \log\left(\frac{a - b}{a+1}\right) \quad \text{or} \quad \log(a+1) = -\log\left(\frac{a - b}{a+1}\right)
   \]
   For the first case:
   \[
   a+1 = \frac{a - b}{a+1} \implies (a+1)^2 = a - b \implies a^2 + 2a + 1 = a - b \implies a^2 + a + 1 = -b
   \]
   For the second case:
   \[
   \log(a+1) = -\log\left(\frac{a - b}{a+1}\right) \implies \log(a+1) = \log\left(\frac{a+1}{a - b}\right) \implies a+1 = \frac{a+1}{a - b} \implies (a+1)^2 = a - b \implies a^2 + 2a + 1 = a - b \implies a^2 + a + 1 = -b
   \]

6. **Solving the system:**
   Combining \( 5a + 3b = -3 \) and \( a^2 + a + 1 = -b \):
   \[
   b = -a^2 - a - 1
   \]
   Substituting into \( 5a + 3(-a^2 - a - 1) = -3 \):
   \[
   5a - 3a^2 - 3a - 3 = -3 \implies -3a^2 + 2a - 3 = -3 \implies -3a^2 + 2a = 0 \implies a(2 - 3a) = 0
   \]
   Therefore, \( a = 0 \) or \( a = \frac{2}{3} \).

7. **Finding corresponding \( b \):**
   For \( a = 0 \):
   \[
   b = -0^2 - 0 - 1 = -1
   \]
   For \( a = \frac{2}{3} \):
   \[
   b = -\left(\frac{2}{3}\right)^2 - \frac{2}{3} - 1 = -\frac{4}{9} - \frac{2}{3} - 1 = -\frac{4}{9} - \frac{6}{9} - \frac{9}{9} = -\frac{19}{9}
   \]

8. **Checking the conditions:**
   For \( a = 0 \) and \( b = -1 \):
   \[
   f(0) = |\log(0+1)| = 0 \quad \text{and} \quad f\left(-\frac{-1+1}{0+1}\right) = f(0) = 0
   \]
   \[
   f(10(0) + 6(-1) + 21) = f(15) = |\log(16)| = 4 \log 2
   \]
   For \( a = \frac{2}{3} \) and \( b = -\frac{19}{9} \):
   \[
   f\left(\frac{2}{3}\right) = \left|\log\left(\frac{2}{3} + 1\right)\right| = \left|\log\left(\frac{5}{3}\right)\right|
   \]
   \[
   f\left(-\frac{-\frac{19}{9} + 1}{\frac{2}{3} + 1}\right) = f\left(-\frac{-\frac{10}{9}}{\frac{5}{3}}\right) = f\left(\frac{10}{15}\right) = f\left(\frac{2}{3}\right)
   \]
   \[
   f(10\left(\frac{2}{3}\right) + 6\left(-\frac{19}{9}\right) + 21) = f\left(\frac{20}{3} - \frac{38}{3} + 21\right) = f\left(-6 + 21\right) = f(15) = |\log(16)| = 4 \log 2
   \]

The final answer is \( \boxed{ a = 0, b = -1 } \) or \( a = \frac{2}{3}, b = -\frac{19}{9} \)