I do not know what machine you have- or what needles you have that are size 4.0...my long arm machine needles range in size from 14 up to 19; * 14, 16, 18 & 19 are the sizes I keep in stock, use * depending on the thread being used. Visit the Superior Threads website- they have free charts you can print off telling you which needles to use for which threads you are using- I've found when I buy thread from them the recommended needle *for domestic and long arm machine* is printed right on the end of the spools/cones too* which is nice- but the charts will tell you what the different threads are best used for- and what size/type of needle to use with that thread. *threads are listed by type/fiber content/ weight*