When first learning how to make your own wine it is important that you don't become too impatient, however since the wine has been bulk-aging for 4 months, I would say you've been patient enough. It would be fine to filter your wine at this time.
One of the more common wine making tips I share with beginning winemakers is:
"Never filter a cloudy wine. The wine should be done fermenting and look clear before filtering".
You can verify that the wine is done fermenting by testing it with a wine hydrometer. You should be getting a test reading of .998 or less. For more information about this you may want to take a look at the article, "Getting To Know Your Hydrometer" listed on our website.
A wine filter is not designed to remove visible particles from a wine. A wine filter is designed to take out very fine particles, smaller than the human eye can see. This gives the wine a beautiful, polished appearance. It brightens the wine.
With this in mind, it is important to make sure that all the sediment that can fall out of the wine on its own has done so, otherwise the extremely fine filter pads that are used in the wine filter will clog up very quickly.
If you are making wine from wine concentrates, the sediment will fall out fairly easily on its own in a week or two, but if you are making wine from fresh grapes or some other fruit, getting all the sediment to drop out can sometimes be challenging. For this reason, it is suggest that you treat the wine with bentonite before filtering.
Speedy bentonite is a fining agent that will help speed up the natural falling-out of the sediment so you can filter your wine sooner and more efficiently. To learn more about fining agents you may want to reading the article, "Using Finings To Improve Your Wine".
You will also want to rack the wine off the sediment before filtering the wine. This will eliminate the chance of drawing sediment into the wine filter.
There is another, more simple, way to answer the question: When should I filter my wine? Filter the wine when it is ready to be bottled. Make it the last step the wine goes through before it is put to rest in the bottle. There is no advantage to filter the wine before that time.
Happy Wine Making,
Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.