Hi

As you mentioned here that packets are taking path from both R3 and R4 because of same metric . The reason here is that the whenever a cisco router forward traffic it will match the destination ip address in routing table and in which longest prefix match , it will send packet by that path . In this task , R1 and R2 receive R5 loopback on /24 from both R3 and R4 , so whenever they will generate packet , the router will send the packet by both of the routers because both of the path are available in routing table with same AD , METRIC and PREFIX-LENGTH . 

How can summarization affect what path the packets take ?

Yes Summarization can effect that what path  will be taken by R1 and R2 to reach loopback of R5  . In this scenario , both R3 and R4 are advertising R5 routes on /24 with the same metric . But if we will send the summary of R5 loopback from R3 than the path is taken by R4 , because R4 will advertise individual routes to R1 and  R2 . So longest-prefix will be match in R4 advertised routes .

Is metric affected due to summarization ? 

There is a simple rule in summarization  that whenever a router will generate summary route , than it will advetise metric of a prefix which one have lowest metric  . But in this task , on R3 every loopback of R5 have same metric .

To perform this task , you can send summary of R5 loopback from R3 and R4 will advertise individual routes on /24 , so  R1 and R2 prefer path by R4 because of longest-prefix match . 

Hope it will helpful for you . 

Thanks