sed - copying every nth line to a new line -

i have txt file need copy 1st line of every 4 lines , print onto 3rd line of every four. , print new txt file. e.g

@cr5sm:00004:00029 ttttctctttctttctt + >>>/>@99419baaabb @cr5sm:00005:00026 attatagagggatag + ;969999999-4;bb 

change this:

@cr5sm:00004:00029 ttttctctttctttctt +cr5sm:00004:00029 >>>/>@99419baaabb @cr5sm:00005:00026 attatagagggatag +cr5sm:00005:00026 ;969999999-4;bb 

i have tried using awk cant seem find correct commands this. have solutions? thanks

using awk:

$ awk '/^@/{a=substr($0,2)}/^\+/{$0=$0 a}1' file @cr5sm:00004:00029 ttttctctttctttctt +cr5sm:00004:00029 >>>/>@99419baaabb @cr5sm:00005:00026 attatagagggatag +cr5sm:00005:00026 ;969999999-4;bb 

you can redirect output file saying:

awk '/^@/{a=substr($0,2)}/^\+/{$0=$0 a}1' file > newfile 

• we use substr function capture lines start @ second character onwards until end of line.
• we lines start + (notice escape since meta-character). once find line, append our captured line existing line.
• 1 @ end allows print lines.