Common (French) lilacs need sun......much shade and they simply won't bloom well. And most do get to be good sized shrubs. Attempting to keep one to half its 'normal' size is likely also going to result in an altered bloom cycle, as these shrubs bloom on growth that was produced the previous season -- excessive pruning required to keep the plant at half size will remove the potential for flowering and if a lilac cannot flower, what's the point?

Unless you have the space to allow it to grow to its full size and in a sufficient amount of sun, it may make more sense to gift the shrub to someone who can put it to proper use. There are dwarf types of lilacs - 'Tinkerbelle', 'Miss Kim', the new rebloomer 'Bloomerang', etc. - one of these may be a better choice for you as most will naturally stay quite small, more within the 4'x4' range you need.

I agree with gardengal, but without knowing your space I will suggest something that would work just fine, if you were inclined. You can select three trunks and prune out everything else. Keep doing so (i.e., pruning out suckers and leaving just the three trunks). You can also prune out the growth on the trunks themselves leaving just the growth on the top third, and even this can be thinned. Done properly this can look perfectly natural and in some senses is even better looking, in my opinion, since the sinuous, muscular smooth grey trunks of lilacs are one of their most attractive features and they last all year long if you make them the focus.

That said, it is your garden and you may or may not want to have that look or that work. If you want a 4 by 4 shrub in that spot, this isn't it and you should find someone who does want it.

(no point putting it in shade, though.)

agree with gal.. full sun or nothing ...

lilac set their buds in late summer ... if you prune for size.. you are likely to cut off all the buds for the next year ..

link below to pruning of lilacs ..

i think you will have a hard time keeping this potential monster to the size you wish ... and have the bloom you wish ...

regift it if possible ... or just give it a go .. and enjoy it.. until it makes you get rid of it ...

was it a personal gift.. or something thrown into an order ... one is much easier to regift ..

ken

Here is a link that might be useful: renovation pruning of lilacs