What is the pattern? Is this

What is the pattern?

Is this a pattern where ply is used to refer to the thickness of the yarn? If so, 2-ply is what I would call lace-weight and 4-ply is what I would call fingering weight. Although all yarns could be knit to a variety of gauges, lace-weight in particular may be knit to different gauges since it is used so much in lace patterns. Knowing what you want to knit would be very important in providing any suggestions.

Or is this a pattern that calls for a yarn that happens to actually just have 2 -plies and you have a yarn that happens to have 4-plies. If these two yarns are of similar thickness, you may not need to make any adjustments at all as long as you can obtain pattern gauge with your yarn.