What bonus is better +4 Defence or +10 Dodge?

Depends on the unit.

I'll take the +10 dodge.

It depends not only on the unit, but on the enemy units, too.

survival bonus from dodge would be:

10 / (enemy accuracy - your unit dodge(wihout bonus))

survival bonus from defence would be:

4 / (enemy damage + your unid defence + 4)

So at the start of the game defence is probably much better. At the end dodge will be probably better, unless AI has developed enourmous accuracy via technology.

When Weapons and Armor values are lower (say 10 or less), the +4 to defense is HUGE.  In later stages of the games, when such values are crossing the Dozens threshold, that +4 is much less significant r.e. reducing damage.

So I'd recommend +4 defense for lightly armored units, and less powerful weapons, but with heavily armored units, and/or with the later game more powerful/uber weapons, the +10 dodge might be more helpful, especially if they have other Dodge bonuses in play as well.

Yeah, good advice in the above: On a early game unit with 0 defense the +4 defense would make it much more powerful and definitely be the better option. If said unit already had 20 defense the +10 dodge is far better.