D&D 5E - How to add "Pact of the Talisman" to skill and save rolls.

So with the release of the Unearthed Arcana Class Feature Variants , I'm letting my players use the alternate character options. One of my players has decided to play a warlock and use the Pact of the Talisman. This allows them to add a d4 to any skill that they are not proficient in and, if they pick the Eldritch Invocation, allows them to add a d4 to saving throws they aren't proficient in. Is there any way to do this on a character sheet? I'm using the base Roll20 5E sheet.

Yes, on the options page (the gear symbol on the sheet) there are several checkboxes on the far right top section that say "Show Global Modifier for XXXX" Check the box for Skill Checks (and when appropriate for Saves) and a new line will appear on the main sheet where you can add in the d4. The one for skills appears in the tool proficiency section, but works for any skill/ability check.

Nice, this works great. Thank you. Is there possibly a way to make it so it works like Jack of all Trades? So it only applies to non-proficient skills?

I don't think you can add a random variable like that to the skill itself. Those numbers work to change the target number for the formula (even Jack of all Trades works that way). You can only add a variable amount in the global mod, which adds to the die rolled instead of changing the target number. Your best bet would be to create a custom Character Ability that rolls skill rolls the way that character needs to be rolled (sidestepping the sheet macros altogether).