Since this is never explicitly spelled-out in the core rulebooks as far as I can see, it’s probably worth a post. (I originally worked this out in a comment thread that I doubt anybody but Russell is reading by now…)
The default assumption in Savage Worlds is that typical Joe or Jane Citizen characters have a d6 in each Attribute, and a d6 in each skill that’s relevant to their profession and daily life. Character generation gives you enough points for a d6 in every stat, and you shouldn’t put a d4 in one unless you intend that your character be wimpier than an average adult at it. You shouldn’t start with a d4 in a Skill unless it’s something the character hasn’t had much practice at up until now.
In the SW:Explorer’s Edition rulebook, the evidence for this is slim, but it’s there: the Youth Hindrance and the Elderly Hindrance both represent less-than-physically fit adult specimens, and neither drops any Attribute below a d4. An 8 year-old girl has a Strength at minimum of a d4, as does a 90 year-old grandmother; they could be stronger… even a lot stronger, but they can’t be weaker by the core rules. There are Hindrances that can give you an effective die-roll even worse (e.g. Anemic, which subtracts 2 from many Vigor rolls), but d4 is the rock-bottom for an Attribute.
The Toolkits add more direct evidence: the “Typical Citizens” entries in both the Science Fiction and Fantasy toolkits have a d6 in each Attribute. The Pulp toolkit doesn’t have a citizen entry, but has a fair number of everyday sort of archetypes such as Snitches, Typical Mechanics, Nosy Reporters (as distinct from Plucky Reporters) and they all fit the pattern of at least a d6 in every Attribute, with only notably stupid characters such as Thugs having a d4 Smarts, or notably young characters such as Wise-Ass Kid having a d4 Strength and Vigor. Even Professors are assumed to have a d6 Strength and Vigor.
The Toolkits also provide the only real evidence of the assumptions about what’s a typical Skill level. The SW:Ex core has few examples of normal people, and orcs and cannibal islanders are just different enough that while they might represent typical opposition to the heroes they aren’t necessarily indicative of what the random soda-jerk, janitor, or dung-spattered peasant is capable of. Basically, Citizens in the SF and Fantasy Toolkits have at least a d6 in every skill that’s relevant to their daily lives, and a d4 in either Fighting or Shooting depending on the typical weapon of their culture (and Guts, if the setting uses it). What they don’t have is very many skills: Notice, some Knowledge Skill representing their trade, and either Driving or Stealth, plus the aforementioned combat and Guts, and that’s it.
While the point-buy system encourages PCs to dabble in a lot of skills (adding a new Skill at d4 after character creation is as expensive as raising two other skills by a die type), it seems pretty clear from the supplementary material in the Toolkits that having merely a d4 in a Skill isn’t intended to represent a competent practitioner. A random NPC that you meet who has that skill as his trade will likely have a d6 in it. Now, because PCs are Wild Cards, their chance of success on a d4 plus the Wild Die is significantly better than the random Extra’s chance of success on a d6 (62% vs. 50%), but my interpretation would be that represents something like raw talent or luck, not training.