Why do they need to be stored if they're being passed to an external application? If you're storing them then just make them individual fields otherwise you'd have storage for 5 fields instead of 4 and just duplicating data, which both situations are bad practice.

You should implement the processing into 4 individual fields when you pass it on to your external application. How are you passing the data to an external application would determine how doable this is though. If for example using CB Auto Actions it should be completely doable to split it using format functions.

In that case 4 individual fields makes sense or contacting the developers of Joomdle to see if they'd consider implementing support for multi-select and multi-checkbox fieldtypes, which are always joined by |*| in storage, to better allow for passing data. As for lining 4 individual fields up to look like you a multi-checkbox you won't really be able to do that since single checkbox and multi-checkbox just completely display differently.

It can do that using a Field action, but again I highly recommend you do not do this. You're storing a field twice. This is a situation of either change the field setup or improve Joomdle (this would be the best approach as appears Joomdle just flat out doesn't support multi-select fieldtypes). Regardless if you insist on doing this the below can be used to change a fields value.

Global
Triggers: onAfterUserRegistration, onAfterNewUser, onAfterUserUpdate, onAfterUpdateUser
Type: Field
User: Automatic
Access: Everybody
Action
Field: CHECKBOX_FIELD
Operator: Set
Value: [cb:if FIELD_NAME=~"FIELD_VALUE"]1[cb:else]0[/cb:else][/cb:if]

Select your checkbox field for CHECKBOX_FIELD. Replace FIELD_NAME with the name of your multi-checkbox. Replace FIELD_VALUE with the value in that multi-checkbox that represents CHECKBOX_FIELD. It should toggle its checked state based off the multi-checkbox. Use the "Add Rows" button and to this for each value of the multi-checkbox field that you want to pass to individual checkbox fields so this can all be done in a single auto action.