After laminectomy for removal of spinal tumor at L3-4, will the spine still have the same strength for weight bearing and exercise as before surgery?

It will take some ti. Laminectomy does weaken the spine for some time. You will have to let the area heal over a period of 3-4 weeks. Then gradually increase your activity to strengthen your back muscles. Walking is a good way to begin this process. A Physical therapist can better guide you in muscle strengthening exercises for your back. Over a period of 6 to 12 months you will regain most of your strength. But go slo.
Should be fine. Of course with a laminectomy they're removing a portion of the vertebrae. However there still stability and since no fusion was required you should be fine. .