I've had 5 CT scans over the past 5 yrs. 4 abdomen, 1 head. Has this increased my risk a lot for lymphoma or leukemia? What if I need more?

Underlying disease? The bigger question is, why have you had so many ct scans over such a short period of time.Do you have some sort on underlying disease that requires so much intervention? Since you've had 4 abdominal ct scans , do you have ibs.
Your total radiation. Dose is still quite low and the time frame is long. That said, it is worth asking about the necessity of tests when additional scans are considered.
Unknown. Most of the data on this is theoretical. You could use the analogy of smoking. Yes it causes cancer but how many packs do you need to smoke before you get it. Same with radiation. It also somewhat depends on the doses you got. Not all ct scanners are created equally and not all radiologists pay as close attention to dose as others. All in all, your risk did not increase a ton.