Hullo Samfire,
I've no idea where you can get an adaptor 4 inches into whatever size your vac hose is, but here's what I did for my new scroll saw (which has a ridiculously huge outlet!):
- 2 bits of 22 mm scrap ply (adaptor needed to be quite long for this particular job);
- screw/pin/glue the 2 bits together then right through with a hole saw nearest the INTERNAL diameter required;
- small rotary sander (in pillar drill) SLOWLY removing a little by little from the internal bore until a tight fit on the vac hose;
- scroll saw (jig saw would do) to cut out the outer diameter of the ply adaptor;
- ditto small rotary sander to SLOWLY remove material until a tight fit on the end of the adaptor to be fitted to the tool;
- in my case (temporary joint required so vac can be decoupled from time to time) 3 x small RH wood screws on one end, suitably-sized Jubilee Clip on the other (TIP: always choose the nut-headed fastening screw type of clip, NOT the screw driver slot only type of clip - they're carp IMHO);
- if removal from vac not required, bathroom silicon/polyurethane glue/whatever you have, to bond hose end to ply adaptor and tool, supported while drying overnight with gaffer tape/small screws.
Worked OK for me, and leak proof as far as I can tell. Just FYI, OD of my adaptor is 79 mm, ID 49 mm. Finished with a coat of varnish ('cos I had it) & to look decent, but has no practical function.
HTH. If you need a pic just ask, but it's VERY simple n doesn't take long.
AES