When calculating a confidence interval for a proportion, you should keep the following in mind:

The confidence levels can be very inaccurate unless your sample size sample is very large.
The actual confidence level is often less than the confidence level you specify.

This website also computes the "plus four" interval.
Statistical literature suggests that the plus four interval yields better results
than the usual large-sample interval.
Experts in statistical practice recommend that you use the
plus four interval for estimating a proportion when
the confidence level at least 90% and the sample size is at least 10.