Hi Kamana
If you have ArcGIS you can use the Reclassification tool as it allows several classification techniques. The most common one is to classify equally and I saw some papers using the natural breaks classification. For the equal classification, many papers follow (0-0.2 not suitable, 0.2-0.4 least/marginal, 0.4-0.6 moderate, 0.6-1.0 highly suitable/optimal). If you will publish a paper out of your work, some journals will be ok with that while others might ask for an ecological explanation for the classification especially that you have specified the suitability degree, so you might put the range of numbers for each category/color instead of words and keep the legend title as something like relative habitat suitability or suitability ranks.
For the natural breaks method (aka Jenks natural breaks), you can specify your desired number of classes for example 4. It works by creating "natural breaks that are inherent in the data itself where it reduces the variance within classes and maximizes the variance between classes" (quoted from the gibbon paper in the attachments). I personally used this method in some studies and I found the classification satisfying from visualization and ecological perspectives (where I explore any possible relation between the natural thresholds and the environmental variables that are strongly affecting my species distribution). Some studies made an extra effort and conducted a statistical method in R to help them determine the number of classes instead of determining it themselves -just to be more valid- So if you are good with R you can try that too.
I hope my answer is useful