Personally if I was you I would grab those 2 Plossl's in the classifieds , the 9 and 15mm GSO Bintel's for $40 and use them with the existing eyepieces you have Giving 9 , 10 , 15 and 25mm and with the Barlow 4.5 , 5 , 7.5 , 9 , 10 , 12.5 , 15 and 25mm's a very good spread of magnifications .
The 4mm you got will only be used on those 2-3 times a year nights when you can use very high power . On that a good GSO 32 or 40mm Plossl would round off a set that will last you years , well into your next telescope
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There is nothing wrong with what you have ( good scope with totally usable eyepieces ) and those 2 will compliment them very well , most of all enjoy what you have and don't worry about spending 2-400 $'s on 1 or 2 eyepieces at this time , as others have said Plossl's are great eyepieces .
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Oh yes
to IIS .
Brian.
ps I have a 11 , 15 , 20 , 25 , 32 and 40mm set of TV Plossl's and a 2x Celestron Ultima Barlow all got from the classifieds here over time and find I use this set more than all my others , by a big margain .