Now I have some questions that how do you reduce the estimated standard deviation.My true parameters beta=c(0.2,0.8,0.5,-0.5), and then when I use a stan model to estimate.The result is following:

I run four chains, all convergent, and the result is shown above.But I find that the standard deviation is a little bit big.so in order to reduce the standard deviation and improve the accuracy of the estimate, how do we solve this,thank you