Posted: 01/07/2012 at 15:28
If you cut lilac back in October, you cut off all the wood upon which it could flower the following Spring. The time to prune lilacs is immediately after flowering, so that they have a chance to have hardened shoots for the next year upon whch they will flower.
If you cut it back very hard, i.e. ground level, you will get lods of smaller 'trunks' instead of the central one I am assuming you have now. Some folk like this and do so, you just want to think about it before you begin.
Some plants do take a long time to begin to flower, much will depend whether it is a grafted tree or not. We have a clematis that took 7 years before it fldowered, and does so prolifically every year now. Patience is often a virtue in gardening. 4 years is not long in a trees life, especially if it is not a grafted specimen.