... different manufacturers use different terminology for their ND filters. For some, "ND4" means 4 stops, but for many of them "ND4" means only 1.3 stops darkening.
No filter manufacturer in this world would call a 1.3-stop or 4-stop ND filter "ND4".
There are two ways of indicating the density of an ND filter---either by specifying the exposure factor (e. g. "ND 4×" which means two f-stops) or by specifying the logarithm of the exposure factor (to the base of ten) which is particularly common with the denser ND filters ("ND 4.0" means an exposure factor of 10^4 = 10,000× = 13 f-stops). Each f-stop adds 0.3 to the logarithm because log 2 = 0.3---so ND 4× = ND 0.6 = two f-stops. ND 8× = ND 0.9 = three f-stops. ND 16× = ND 1.2 = four f-stops. And so on ...
An ND 4.0 filter (i. e. 13 f-stops) is almost opaque to the eye and costs considerably more than an ND 4× filter. A ten-stop ND filter would be ND 3.0.