$1 lit\Rightarrow m_{dd}=1000 gam\Rightarrow m_{HCOOH}=4,6 gam\Rightarrow n_{HCOOH}=0,1 mol$ $HCOOH\rightleftharpoons HCOO^-+H^+$
$x x x$
$\Rightarrow \frac{[HCOO^-].[H^+]}{[HCOOH]}=K_a\Rightarrow \frac{x.x}{0,1-x}=1,5.10^{-4}\Rightarrow x\simeq 3,87.10^{-3}$
$\Rightarrow \alpha =\frac{x.100}{0,1}=3,87\%$
$\Rightarrow [H^+]=3,87.10^{-3} M$