Welcome aboard Af. You might want to check the Reviews section for 4.5" reflectors...
I'm guessing the 4mm is labeled SR - if so save the barrel for a webcam project
Have the others nylon or metal barrels? Anyway I'd say save your pennies for the momment.
Either way a 32mm plossl will max out the field of view for your 1.25" eyepiece tubel, Bintels own brand of GSOs are only $39 or you could try a better Meade series 4000 for $69, or for double that a Televue 32mm plossl.
You can get some more magnification and the same field of view by paying more and getting a superwide angle or ultrawide angle - a 20mm or so superwide or ultrawide like the meade 4000QX or GSO superviews.
A good barlow will help if yours is a plastic cheapie, say X2 or 2.5
But for the momment why not just suck and see for a bit.
regards
Two good places to shop would be (amongst others) Bintel and Andrews Communications.