According to Bluemix dashboard, total number of service instances I permitted to use is 4, but my colleagues' are 10. What cause this difference?
Is there a way to increase the total number of service instances on Bluemix?
Number of service instances depends on your org quota.
Type cf quotas in terminal to find out quotas available.
cf org your-org-name would show your current quota.
It sounds like your are on Trial, Check with your colleague on how he got more quota than yours.
If you think that you are assigned incorrect quota , please contact Bluemix project office team to investigate the discrepancy : http://ibm.biz/bluemixsupport --> Select ID+Login
cf tool can be downloaded from here : https://github.com/cloudfoundry/cli#downloads