Usando colar
$ paste -d' ' - - - - <file
16:20:03 Buy DRIP AMEX 500 13,51 USD
16:05:10 Sell SQ NYSE 100 36,32 USD
15:48:52 Sell NXTD Nasdaq 500 4,99 USD
15:48:52 Sell NXTD Nasdaq 500 4,99 USD
15:46:07 Buy SOXL AMEX 50 147,7209 USD
15:40:20 Buy TEUM AMEX 1 700 1,36 USD
15:40:19 Buy TEUM AMEX 300 1,36 USD
Usando sed
$ sed 'N;N;N; s/\n/ /g' file
16:20:03 Buy DRIP AMEX 500 13,51 USD
16:05:10 Sell SQ NYSE 100 36,32 USD
15:48:52 Sell NXTD Nasdaq 500 4,99 USD
15:48:52 Sell NXTD Nasdaq 500 4,99 USD
15:46:07 Buy SOXL AMEX 50 147,7209 USD
15:40:20 Buy TEUM AMEX 1 700 1,36 USD
15:40:19 Buy TEUM AMEX 300 1,36 USD
Usando o awk
$ awk '{line=line " " $0} NR%4==0{print substr(line,2); line=""}' file
16:20:03 Buy DRIP AMEX 500 13,51 USD
16:05:10 Sell SQ NYSE 100 36,32 USD
15:48:52 Sell NXTD Nasdaq 500 4,99 USD
15:48:52 Sell NXTD Nasdaq 500 4,99 USD
15:46:07 Buy SOXL AMEX 50 147,7209 USD
15:40:20 Buy TEUM AMEX 1 700 1,36 USD
15:40:19 Buy TEUM AMEX 300 1,36 USD