Firstly, you can always upgrade items by paying the difference (DMG p288):
A creator can add new magical abilities to a magic item with no restrictions. The cost to do so is the same as if the item was not magical. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 sword.
Flame Tongue is slightly different because it has a unique ability, so that there technically is no +4 Flame Tongue in the books to upgrade to, but according to the FAQ (p.63), you can upgrade it if the DM gives permission:
That said, a DM willing to allow leeway in item creation could allow a character to upgrade such an item's capabilities, using the same guidelines as for improving any other item (see previous question).
Based on the examples given in the FAQ, Flame Tongue is a 20,715 gp +1 flaming burst longsword with a unique ability, and costs 2,400 gp more than a standard +1 flaming burst longsword.
You can therefore upgrade your weapon by paying the difference between a +1 flaming burst sword and the item you want to upgrade to.
As for level requirements, being level 12 and able to craft a weapon of up to +4, that means you can give the weapon an enhancement bonus of up to +4. That's actual enhancement bonus, so you can make a +4 weapon with as many special abilities as you like until you hit the hard limit of +10.
Creating a magic weapon has a special prerequisite: The creator’s caster level must be at least three times the enhancement bonus of the weapon. If an item has both an enhancement bonus and a special ability the higher of the two caster level requirements must be met.
This seems to imply that you also need to be level 12 to put a +4 equivalent special ability onto a weapon, though it doesn't specifically say this, so it's a little vague. It doesn't matter at all in your case since the only special ability higher than +4 is vorpal.
In your case, the +1 Flame Tongue is priced as +3 (Flaming Burst property is +2). This means if you upgrade it to +4 Flame Tongue, it is priced as a +6 weapon (plus the 2,400 gp for its special property). You can attach up to +4 in additional special abilities before you hit the +10 hard cap.