Hans,
The differences noted by Michael and Mark above are true.
Though, to convert the Tamiya F4U-1a to a F4U-4, wouldn't be too great, given that it doesn't sound like total scale accuracy is your goal, just something that looks the part.
It can be done, with a little work. The canopy's in the 1a kit also include the 1d sliding portion of the canopy, which was the same as the -4, easy!
The windscreens were flat in the front, as the curved portion was replaced with bullet proof glass. Sanding the front portion to match, and polishing it out should capture this. Also, depending on your audience, they may not notice.
It's true on the engine. The R-2800 was larger, however the size of the cowl is pretty much the same. Here's where it would get a bit scratchy. Since the cowling flaps on the -1a kit are open, you can take stretched sprue, or tubing to represent the exhaust pipes that came out midsection (2 on each side), not a big deal. One would have to remove some plastic where the exhausts exited on the fuselage behind the cowl flaps, and scribe some lines to show that addition. Going good on the exhaust streaking typical of this variant would distract the eyes from the actual exhaust mating regarding the depth you go. Still not too bad.
The -4 does have an airscoop on the front of the cowl underside. This would have to made using something like sheet styrene and some putty. This would then need to smoothed out and blended into the cowl. depending on your skills this would be a challenge.
Do all that, get some nice Korean Way decals, go with a Gloss Sea Blue finish (this paint did not fade!), and then go to town on weathering it hard, Marine F4u-4's looked pretty beat. Then just source a 4 blade Hamilton Standard prop. Monogram/Revell P-47, their P-61 Blackwidow (does anyone build this kit?), or even their B-26 Marauder would be good inexpensive sources.
Ideally, if you have the Tamiya P-47M in your stash, the engine crank cover with the magnetos would be an enhancement, and these kits also have all the props used by the P-47, so you would have the 4 blade Hamilton standard prop! You can use them, and still make a great P-47 with what's left in the box!!
That's my 5 cents worth, and how I'd go about it. Or, you pick up the older Hasegawa F4U-4, or their -5N, at a pretty reasonable price (given its age). Hope that helps!
Stay safe and have fun!!
John