Verified answer

All you have to do is go to the cave in dewford city and talk to a hiker inside the cave. He'll give you the HM05 flash.

To use the HM flash, you have to beat Brawly first.

Dewford City

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1. Battle the fishermans in the north-west area.

2. Talk to a hiker to get the HM, Flash.

3. Go to the Gym.

rest of walkthrough = http://uk.faqs.ign.com/articles/715/715209p1.html

HM4 flash is located in dewfentor town.where brawly is were yocan also provebly find regi ice