If the lavender plants have been potted for 3-4 weeks and you don't see any new growth, they're probably dead. Bare root perennials need to be planted right away, which you did. But if potted, they've also got to be kept somewhere cool. Our homes are too warm, but a breezeway or unheated garage works well. Move the pot to a shady spot outdoors and watch for any changes over the next few weeks. If you see no growth and are ready to throw them out, gently lift the plants out of the soil and look at the roots. Any fleshy white roots are signs of life, so you can replant them and give them more time.