Wiring them all into a single battery bank is OK unless the batteries are significantly different in AGE.
If one pair has been used, and the other pair is new, the older pair will draw down the new pair and reduce their lifespan.
The greater difference in age the more this matters. For example, if the batteries are 6 months different I'd say use them and don't worry about it (purists will scoff at this, but what the heck, it will work). However, if one pair is 2 years old and the other is new then it's time to consider buying 2 more new batteries.
Here's a link to some more battery info
*Added on edit* You didn't ask but...
With an inverter that size you may want to see if you can squeeze in six batteries, four is fairly marginal for that size inverter if you plan to use it very much at all.