How do you reduce the estimated standard deviation

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 in order to reduce the standard deviation and improve the accuracy of the estimate, how do we solve this,thank you

To reduce the standard deviation of estimates you can either increase the sample size or use smaller priors (or both)

Narrowing the scale of priors will only yield narrower posterior scale if the data are consistent with the data. I’d avoid that kind of tinkering as it starts to feel like the outcome-targeted prior specification that the more Bayes-naive accuse Bayesians of.