If you aren’t seeing a lot of items due daily and feel like you can tackle new words (in addition to your daily queue of items to review), I would say you should proceed on to HSK4! The only reason I would suggest to not add new items would be if the amount of items to review each day are high, adding new words on top of that will make the queue even larger day to day.
You can hop on the legacy site at https://legacy.skritter.com, and then go into your HSK3 list for instance, if you click on a word, it will show it’s scheduling information including when it’s due next for review.
You could also use the calculator at: https://legacy.skritter.com/study
You can click the “Click to calculate” text, and then click on the calendar, which will show you how many items are due over the next 8 weeks. You could use that value to assess whether you would want to be adding new words or just focusing on reviewing your current lists for awhile.