Question 8gb with 4gb ram

[OtakuSensei]

as i post last week my ram is 4gb with unknown mhz. i tested it with cpu-z then it says 665.1mhz then changes in second for 1300mhz then goes back to 665.1mhz and the uncore is always 3000+mhz. so I wanted to change my ram to 8gb is it ok to insert my 8gb ram to first slot then the 4gb ram is in second? or is it good if 8gb ram only?

 

[OtakuSensei]

my laptops is dell vostro 3490 with i5 10th gen and 4gb of ram