To be completely successful with your real nappies extra boosting will be required at some point.  How much a baby wees varies considerably, from baby to baby and from stage to stage.  Most newborns wee little and often and you are changing the real nappy, not because it is wet through but because it is soiled, therefore boosting is not needed.  However often at around 4 months a baby is settled into a routine, still not on solid food and getting hungrier, so drinking lots and the washable nappy may need changing every hour instead of every 3 or 4 hours.  So the best solution is to boost the nappy to make it more absorbent and last longer.  Simple.  Boosters can be made from bamboo or cotton, or even micro fibre.  I have found that bamboo is the best, slimmest and most absorbent so generally recommend using that.  But anything can be used including muslins or prefolds.  You should find that having a few bamboo boosters in your kit makes dealing with these changes easy.