Take a look at the soybeans in this photo. If you were going to apply a fungicide that should be on by R2 stage for maximum effect, are you late?
In this case the answer is likely yes. If the fungicide label says you can treat through the R4 stage, you could still apply it, but you might not get maximum effect as if you had sprayed earlier.
This can be a product-by-product decision so you need to look at labels carefully.
So how do you stage soybean growth? The Purdue University Corn and Soybean Field Guide in pocket edition or as an app for your iPhone, has a series of pictures that can guide you and help you zero in on the right growth stage. If the plant is in full flower with flowers on one of the two uppermost nodes, you're likely looking at stage R2. The 'R' refers to reproductive. The earlier 'V' stages are the vegetative portion of the growth cycle.
Since pods have started to form on the plants in the picture, they would likely classify as R3 plants. By R4 or full pod, the flowers are gone and the pods that are going to form have formed. When seed starts to from within the pods, the plant transitions to R5. The R6 plant has greed seeds in pods on one of the uppermost nodes on the plant. The plant doesn't reach the R 7 or full maturity stage until at least one pod on the main stem reaches its mature pod color, normally either tan or brown. Variety usually determines the color of the pod through genetics.
Stage the growth of soybeans before making any major decisions, such as whether to apply a fungicide and/or an insecticide, and if so, when to apply it.