hda1 thru hda4 are primary partitions. You can only have four primary partitions on a disk. Of these four primary partitions, one of them may be designated an extended partition. Within this extended partition, you may have 63 (64?) logical partitions. Logical partitions are numbered starting at "5" and going up from there (e.g., hda5, hda6, etc.) The extended partition cannot be followed by any more primary partitions (this in reference to physical layout on the disk, not the order of entries in the partition table, which can vary).
So your current setup would allow you to create more logical partitions at a later date (providing you have unpartitioned space in that hda3 extended partition). But you could not create any more primary partitions unless your unallocated space happens to fall (physically) between your current hda2 and hda3. This is probably not the case, unless you specifically partitioned to keep the middle of your disk free. Not that this makes any difference to Linux. Linux doesn't care about this primary/logical thing like Windows does (and Windows can be coerced into "not caring" with a little effort).
Short answer: Yes, you can chop up the remaining free space in your extended partition at a later date. (Just don't plan on installing Windows there, without some work.)